I'm pissed that my ex gf faked orgasms with me
How did you find out about this?
Because I've made my research on this issue and I found out that it's very common
I can understand how you are frustrated by this. It's understandable to be upset when a partner is dishonest with you. The issue is sensitive for women so maybe it is something that concerns her also.
It's not that big of a deal and I'm going to tell her that she is a slut if she is doing it with other guys.
That is not a nice thing for you to do. It's understandable to be upset with someone who is dishonest with you, but I would ask her for the truth. Just because it is very common doesn't mean she faked it.