HELP! Need help with macro that automatically sends email when...

Hi,

I am putting together a database that will track inventory. I am trying to make a set of macros that will allow my excel workbook to automatically send an email whenever any cell in a specific column (actualy inventory) is less than the respective cell in the column adjacent to it (minimum stock). I copied most of this from examples online. Now the email sending macro works fine, I need help with the trigger macro to automatically send the email when the correct conditions are met (a cell in column C has a value less than a column on the same row in D, eg. c3<d3) So far I am losing sleep and getting nowhere... Please help! Here is what I have so far:

Private Sub Workbook_SheetCalculate()
    Dim FormulaRange As Range
    Dim NotSentMsg As String
    Dim MyMsg As String
    Dim SentMsg As String
    Dim MyLimit As Range
    Dim Formulacell2 As Integer

    NotSentMsg = "Not Sent"
    SentMsg = "Sent"

    'Above the MyLimit value it will run the macro
    Set MyLimit = ActiveWorkbook.Ranges("D2:D2000")

    'Set the range with Formulas that you want to check
    Set FormulaRange = ActiveWorkbook.Ranges("C2:C1000")
    
    On Error GoTo EndMacro:
    
    For Each Formulacell In MyLimit.Cells
        With Formulacell
            If IsNumeric(.Value) = False Then
                MyMsg = "Not Numeric"
            Else
                Formulacell2 = Formulacell.Value
              
                
            For Each Formulacell In FormulaRange.Cells
                With Formulacell
                    If IsNumeric(.Value) = False Then
                            MyMsg = "Not Numeric"
                    Else
                        If .Value > Formulacell2 Then
                                MyMsg = SentMsg
                            If .Offset(0, 1).Value = NotSentMsg Then
                                Call Mail_small_Text_Outlook
                            End If
                    Else
                            MyMsg = NotSentMsg
                        End If
                    End If
            Application.EnableEvents = False
            .Offset(0, 1).Value = MyMsg
            Application.EnableEvents = True
                
                End With
            Next Formulacell
        
        End With
    Next Forumlacell
        
ExitMacro:
    Exit Sub

EndMacro:
    Application.EnableEvents = True

    MsgBox "Some Error occurred." _
         & vbLf & Err.Number _
         & vbLf & Err.Description

End Sub

Sub Mail_small_Text_Outlook()
'For Tips see: http://www.rondebruin.nl/win/winmail/Outlook/tips.htm
'Working in Excel 2000-2013
    Dim OutApp As Object
    Dim OutMail As Object
    Dim strbody As String
    Dim rick As String
    Dim orrett As String
    Dim rng As Range

  Set rng = Nothing
    On Error Resume Next
    'Only the visible cells in the selection
    Set rng = Selection.SpecialCells(xlCellTypeVisible)
    'You can also use a fixed range if you want
    'Set rng = Sheets("YourSheet").Range("D4:D12").SpecialCells(xlCellTypeVisible)
    On Error GoTo 0

    If rng Is Nothing Then
        MsgBox "The selection is not a range or the sheet is protected" & _
               vbNewLine & "please correct and try again.", vbOKOnly
        Exit Sub
    End If

    With Application
        .EnableEvents = False
        .ScreenUpdating = False
    End With


    Set OutApp = CreateObject("Outlook.Application")
    Set OutMail = OutApp.CreateItem(0)
    
    rick = "rick.@companyname.com"
    
    orrett = "orrett@companyname.com"
    
    strbody = "Hi there" & vbNewLine & vbNewLine & _
              "Cell A1 is changed" & vbNewLine & _
              "This is line 2" & vbNewLine & _
              "This is line 3" & vbNewLine & _
              "This is line 4"

    On Error Resume Next
    With OutMail
        .To = orrett
        .CC = ""
        .BCC = ""
        .Subject = "Low Inventory Warning"
        .Body = strbody
        'You can add a file like this
        '.Attachments.Add ("C:\test.txt")
        .Display
    End With
    On Error GoTo 0

    Set OutMail = Nothing
    Set OutApp = Nothing
End Sub

F