No, you're not spreading anything. If you don't have anything, you're not spreading anything no matter what you do -- do you have something? As for using your towel more than once, sure, but if others are using it too I wouldn't but not because of STDs, just for hygiene. But please, do dry your whole body. If you're leaving the area near your genitals wet you are possibly giving yourself the chance to get fungal infections. You do want to stay dry down there. Peace.
So, go wash your towel. Not a big thing to worry about here. Why do you think you have an STD? Have you had an encounter or what kinds of symptoms do you have? Getting tested to make sure you don't have an STD is a good idea (for us all). What's the std worry?
The towel is a non issue. That's not a way that std's are spread. Maybe get a second towel though so you only use it every few days rather than a whole week and have worries that you all are sharing towels.
Definitely agree on the towel, AND on getting tested for STDs.
Untreated STDs can cause all kinds of problems, depending on the STD, so please don't wait any longer.
Feel like sharing what's going on? We can probably help.