Ivan_starcraft

Hi everyone, its been a while since i last posted for help, but i guess i really need it
I own a retail business with my girlfriend's exboyfriend...i know its really study, but lately I feel like they are together or she is cheating with me on him, by like very little things i can tell, its hard to explain, but i can say i've got this feeling she and him still have something going on....like am i over suspicious or what? i don't know and that is why i am here today

For example:

The Cash Machine didn't have enough change (Coins) so i went to another store and grab some change, and then i came back to the original store to put change inside then we had this conversation

GF "I saw you take 20 dollars from the cash register to get change "
ME "no i didn't, i took 20 dollars out of the cash register and then put it back in, because i thought i just get the change first and then come back and exchange it "
GF " No i saw you take it....."
and then we started fighting about the 20 dollars
GF " you are going to make it hard to close tonight, because now the number is all messed up"
me" how is the number all messed up, i took 20s out and put 20's back in "
me" you are just concern because you are worried i am stealing from your exboyfriend?? are you guys still together, cheating on me !!! "
and then she got real piss off and started kicking at me, i did nothing and i left.......

What actually happened : I NEVER TOOK 20 DOLLAR OUT IN THE FIRST PLACE, i did take it out, but then i put it back, because i decided to take it when i get the change "
her exboyfriend wasn't here when this happened

MY feelings :
to me its like, why is she so protective about the money, is it because its for the good or me or the good of her boyfriend? OBVIOUSLY IT IS NOT THE GOOD OF ME, because lets just say i did take 20 dollars out and try to steal from the store it wouldn't matter to my girlfriend because if she was on myside. Like she wouldn't care if i actually took 20 dollars with her knowing it that i actually steal, because in the end the 20 dollars is ours, BOTH ME AND HER....
I AM NOT SAYING I ACTUALLY DID, BUT I AM SAYING EVEN IF I DID, she has nothing to be  CONCERN because I AM HER BOYFRIEND, i am not the one being stole from......

PROBLEM: Am i just over reacting? or was her acting correct??? i just want to know if it is my problem and i am over suspicious or THERE IS ACTUALLY A PROBLEM TO HER REACTION