Why does it hurt? :(

So I'm in a fairly new relationship, and we've been having sex for a few weeks now. The main thing I noticed is that I seem to only be able to do it once, because after that it just gets to be too painful. I've never had this problem before and it's kind of irritating. We tried lube and that didn't seem to be the issue. I don't think it's a size issue either, although he is pretty girthy, so idk.

Any advice or tips? Has anyone else dealt with this?