When a guy sticks his thing in it hurts once it's in its goes away I have got tested for std and nothing?

Normal. The male penis can cause pain upon insertion. Once he gets past the opening of the vagina, you likely relax a little and are more "wet". What would help is relaxing before he puts his penis in. Practice relaxing your butt muscles just before and that will help. Also, a little lubrication such as Astroglide can help with that initial insertion to make it more comfortable.