is this an STI?

I have been seeing this guy for about 3 weeks now. He told me he has slept with over 10 people, and I told him I had only slept with one person before (my long-term ex boyfriend).

Yesterday this guy & I had sex for the first time and we didn’t have a condom, so he didn’t finish and neither did I, and then this morning I noticed a fair amount of yellowish discharge, with a slight odour, which I have never experienced before.

I am only 17 so it will be difficult for me to access a doctor and my mum will kill me if I told her,

and I am not sure whether to say something to this guy or not. The sex was pretty rough so I am also experiencing a little pain.

Is it normal for STI symptoms to show up immediately? Or could this be something else?

Please advise, thank you.