Sex long and painfully

Hi. I have hot in a relationship with a guy and we started having sex a couple days ago.
Yesterday, we had sex three times. The first time took less than 20 minutes and it was good. The second one took more than 45 minutes and it was really painful for me. The third one was more than an hour and was so painful for me that I couldn't continue and ask him to stop. He didn't *****.
I like this guy but this stuff made me a little worry about our future sex life. I want to get an appointment with a doctor to find out why sex is so painful for me.
I also had only 17 sex in my whole life and it started several months ago.
How do I feel more comfortable with having sex with this guy?