This morning my penis started to hurt when I got an erection. It stopped when I went flaccid. This happened three times. Was this because my penis is growing?
If you are talking about a dull ache when you first woke up this morning, then most likely the cause is due to having had an erection for too long. The penis can only stay erect for about 30 minutes before a lack of oxygen in the blood inflating the penis make the tissue start to starve. Even after the penis goes flaccid, it still feels uncomfortable and if another erection comes before it recovers, it aches again. If this describes what happened, then it will go away in few hours.
If you are uncircumcised and the pain more sharp and near the end of your penis, then you might be having trouble with your foreskin not folding back as it should. An full erection should have the end of the glans exposed and the foreskin able to freely slide up and down along the shaft of the penis. In general, erections are not supposed to hurt. If your foreskin is sticking to your glans or it is too tight for your erect penis to get through then you'll need to do something to get the problem corrected.
My penis only has a dull pain at the base to the right side of my penis. It only hurts when I get a erection. My penis feels like it is trying to stretch. I am already circumcised.
Most likely your conclusion is correct. Some structures in your penis has grown, but others haven't caught up and thus when you are getting erect it is stretching things a bit. But there is one thing else I would like you to do. Note where the ache is at and then when you return to a flaccid state, see if you feel a lump or hard spot internally at that point. (I'm assuming the ache is on the shaft of your penis.)