My ex-girlfriend had her period last month, now she missed it.

Okay. two months, my ex-girlfriend and I had sex. We had sex for a little bit without a condom on, I didn't bust. Then I put on a condom and we continued. later on, I pulled out, and she finished me off with her mouth. A few days later she has her period. Now this is where the fun begins. A few days ago she said she didn't get her period this month, and saying I am the dad. I told her impossible, we didn't have sex last month we had sex 2 months ago and you got your period. 
TL;DR
>April had sex
>She had period few days later.
>May No sex.
>No period.
>June
>Claims I am Dad.
Wtf? How?