Do I have an infection?

Hey so like most women (I'm a teenager) I've had a yeast infection before, I know what it does, how it feels and what it produces. But recently I have noticed a strong fish like odour from my vagina, and a brown sticky discharge. This is something that my sister has too (I don't know if me being around her the weekend after I noticed this will help) but for some reason she never checked it out or whatever and she smells sometimes. I am very cautious about my hygiene and I wash my private parts with Sanex everyday and you know, I look after myself. I noticed this smell and change after I had sexual intercourse. We had protected sex but I don't know how protected oral sex can be. I searched high and low online for an answer to what is causing this but all the other types of infections have symptoms that I don't. It doesn't itch, it isn't scabing, it's not dry or irritated, no white discharge. My symptoms are purely fishy odour and brown discharge. I really just want insight from someone who has had something similar and how serious it could be. Cause I am not looking to head to the doctors until I know how serious it is. Please help?