Lump near anus after sex??

**PLEASE HELP I'm so worried
So I've recently had sex and 3 days later have noticed my anus has been extremely itchy and red, and there is also a lump close to it. It's also a purple colour around the edge of my anus (keep in mind I didn't have anal sex)
I was itchy around my anus before the sex as well 
Could it be a hemorrhoid or something else?
Cause I've heard it's unlikely to get one when you're 17...