Blog reader's girlfriend started crying during sex. What does that mean?

Hello, I need a serious advice on this matter. Today my girlfriend started crying during sex. 

We've been together for like 3 months and it has been an intimate relationship. You know what I mean.

I must confess though. I hate having sex with condoms. Since we are exclusive, I talked to my girl about it after the first day we used a condom and she agreed we can skip them after her menstrual period - when she is safe. And since then, we have been enjoying ourselves bareback. It feels really good. I always ask her as well if I'm hurting her and if
she is okay while we have sex. She always nods showing her approval.

But last night she started crying out of nowhere. I asked what was wrong, she didn't answer me. I stopped and asked her again what was wrong or if i did something that hurt her and she said "Nothing, please keep going." 

So i tried to start again but she began crying again. I eventually stopped as it was confusing, wore a condom and finished it off.

I asked her afterwards what happened, and she says she doesn't know why she started crying. It's not our first time either we've done it plenty of times before.

Please I need to know what you people think cos I've been thinking about this all day.