increase value by 1 if meets variable..

[Dxmiian]

okay so trying to create a reference log, and when criteria is met (less than 0) I want it to show in another column, Cas1, Cas2 so on and so on.. my problem is getting the number to only increase when criteria is met (less than zero) this is what I have...

=IF(Q5>=0,"",((LEFT($D$1,3))&A1) &A1 is grabbing a 1 from a cell to add to Cas

so for example Q3 has -3 so will show Cas1 Q4 and Q5 are more than zero so wont show a reference, then Q6 is less than zero. how do I get it to lookup the last highest Cas# value and add 1 each time