About pregnancy. This pregnancy case.

My girlfriend is missing her periods since last month her periods were to be on 03.07 but hadn't come and we don't have any sex before that. Earlier also she have irregular periods. After that on 14.07 we have had protected sex but her periods were missing so we got concerned and tested for home pregnancy test and that test was negative. So we concerned a doctor nd she told us repeat the test and if it comes negstive then take the medicine of regestrone. And she has taken that medicine for five days two doses each, last being on 29.07 and after that on 02.08 she had very light periods for two to three days. So we were tensed and tested again on 08.08 and she tested negative again. But she is having some symptoms now like putting on some weight, nausea, headache which is concerning us. Please tell us how can we get sure about this pregnancy case.