What kind of foreplay? Mutual masturbation? Oral sex?
Has your girlfriend ever had any kind of sex with anyone else? Oral sex?
How old are you? If you are in the US, the UK or Canada (and many other countries), you can get checked for STDs without your parents' permission, but you will have to find a way to pay (and a ride if you can't drive yet, etc.).
It could be irritation from masturbation (and there is no "normal" way to masturbate - everyone does it however they like), especially if you're doing it a lot, and not using anything for lubrication, like lotion or a lube.
Lots of things cause bumps on your penis that aren't related to sex - fungal infections, infected hair follicles, things like that. Can you go to your parents, or an older sibling or aunt or uncle you trust, and tell them you haven't had sex yet, but have this bump?
I'm guessing your dad, if he's available to you, has had a fungal infection before. Fungal infections are things like jock itch. If he hasn't had it, he's heard about it. He might be more receptive than you think.