Depo Shot

Hey so I just started my period today and I'm going to get the depo shot tomorrow during school . My mom doesn't know because it's one of those health clinics that you don't need your parents approval. I really am not sure wether or not to get it. I'm 16 years old and I'm quite sexually active but I'm scared of like gaining too much weight or if my regular doctor asks if I'm on any type of birth control and my mom is there do I lie ?