I didn't receive the correct cash back at CVS, what should I do?

I was at the store during my lunch break on Saturday and needed to get $50 in cash so I did two transactions (one for $35 cash back and one for $15) Admittedly, I was in a rush and didn't count the money I got; instead I just threw it in my purse. However, after work I went to buy something that was exactly $50 and I was short $10. I was shocked. But after I got home I looked through my entire purse and still didn't find it. There is no other way that I would've lost $10 except if they had given me the wrong cash back. Will I be able to get my $10 back? How should I best approach the situation when I go in there?