Negative Readings but some possible pregnancy symptoms

Patient: Hi. I took the Depo shot in June, my husband got back from Afghanistan in July so of course we started having unprotected sex. I was due for the next shot around the end of August/beginning of September but I decided not to get it. Around the middle of September I started gaining weight. I’ve had increasingly painful backaches. At work, in the car, at home. They’re nonstop. I’ve been urinating more frequently and had an occasional cramps here or there. A couple of times I’ve taken a pregnancy test but I’m getting negative readings. Is it possible I’m pregnant?? Also I’ve noticed my breasts are more tender, my nipples especially. As for nausea, I’ve always gotten carsick when riding in other peoples cars but I’ve noticed that it’s gotten a bit worse. please help. I want to know if I have a good chance of being pregnant before I make an appt at the military clinic.