I have these red lines that circle the bottom of my penis head. they don't seem to be getting worse, but sometimes after masturbation they get darker.

Here are some ...: Without a look at red lines, it's hard to name it correctly. But what you described likely represents a relatively "constricting deep" skin fold of foreskin right above sulcus coronaris, which has not been infrequently seen and may worsen its appearance after physical stimulation like masturbation. So, have a pro-evaluation; at times, minor procdure like a modified circumcision may be required.