I couldn't cum when she gave a Handjob

Guys, I met this girl who i have been texting for a while now. We decided to casually go on a date and then go back to my place. We hangout at a mall, had food and things were going good. End of the day, we came back to my place, we have already talked about making out. She's a virgin and not yet ready to go all the way in. I agreed to it and we started making out. I enjoyed the way we kissed, it was so good. She wanted to try handjob and BJ as well. I was happy and excited. When she tried to do BJ, she gagged and said she can't do it. I told her it's alright if she don't want to do it as these things happens in your first time. She then started jerking me off. I asked her to play with the my balls while she was jerking me off with the other hand. I showed her what to do and she followed it, it felt so good when she was playing with my balls..

The issue here is, she was jerking me off and i couldn't cum. She kept doing it for around minutes, I was hard af and at some point i started feeling pain in my D. We took a pause and continued after few minutes. However even then I wasn't coming. I felt that I'm losing my erection slowly and she too started telling she's bad in this. I don't want to make her sad and said these happens at the first time. Then I said her I'll teach her how it's done, I held her hands around my D and started shaking myself and soon i came.

I don't understand, why couldn't i cum when she was doing it?

TDLR - She gave me a Handjob but i couldn't cum. I haven't had any issues in the past. Would like to know if this is something I should be worried about.