You said toward the end of your original post, "I haven't always been a nerd"... so at some point you must have been comfortable with your self. It sounds like you are worried not so much about your own sexuality, which is nobody elses business, but much more about how other people are seeing you, and your fears over sexuality are just one way that anxiety is manifesting. What you are describing sounds like anxiety, especially your explanation of the physical response to these thoughts, which seem obsessive. You haven't always been this way. Maybe it's time to talk to your doctor about this?
As for your sexuality - you are the only person who can decide that, or rather, come to terms with that, homosexual or hetero or bi. It is not unusual to question this through early adulthood. I assume you are at an age that many people have sexual identity issues, identity issues in general. I remember incidents in high school that I was "accused of" or asked in an uncomfortable situation whether I was homosexual, by people who were trying to make me uncomfortable. I denied my own sexuality for years, not because I didn't know, but because I knew I was an easy target in our unaccepting society, and they weren't worth the fight. I understand how frustrating it can be, when dealing with these thoughts, to hear people refer to things as "gay" when they mean stupid. It's like a personal insult, fom people who are simply ignorant both of the weight of their own actions and of your feelings. As you develope a stronger self image, things that ignorant people say will bother you less and less. Choose your battles, try to forgive them, they usually know not what they do.
A sort of motto around here is that if something is interfering with your ability to function comfortably, it's time to do something about it. You might find that with anxiety out of the way, the things you are stressing over right now work them selves out.
Good luck, keep posting.