It sounds like you have a few questions and concerns: your mother being disappointed in your decision to have sex now, nervousness about pregnancy, and what happens when you go to the gynecologist for the first time.
Sixteen/seventeen is an appropriate age to go see a gynecologist whether or not you are sexually active, so it may be helpful to go anyway, particularly because they will be able to help you decide on the best form(s) of birth control before you become sexually active. I'm not sure if your parents would have to give consent for something like a hormonal birth control prescription where you live, but if you are worried about your mom finding out it may be good to ask your doctor if that's the case. While pregnancy prevention can't be 100% guaranteed, using two methods of birth control instead of one can offer some peace of mind (e.g. IUD & condoms). I found this article on Scarleteen's website about what to expect at your first gynecological visit: http://www.scarleteen.com/article/bodie ... gist_visit
I can't speak too much to your concerns about your mom because I don't know enough about your relationship, but ultimately she probably wants to protect your health, safety, and emotional well-being. Going to the gynecologist is a proactive step towards doing that for yourself and in the end decisions about your sexuality and sexual health are your own to make. If you don't feel safe/comfortable talking to your mom about sex just yet, do you think there's another trusted adult that you could talk to?
I hope this helps and good luck