Confirmation about pregnancy. Guide us!

Hello doctor.. I want to ask about my girlfriend. She had her period normal in June but then she missed them completely in July and in mid July we have had normal sex after which we were tensed so we tested for pregnancy test kit at home but that shown us negstive result. After that we consulted a doctor in July only and she said to us to be do test after one week and we did it again and it tested negative and then she told her to take regestrone medicine for five days. And she took that and she had periods in beginning of August but very thin periods. After that because of these thin periods we thought to test again for pregnancy by kit but she tested negative. We were relaxed for a while. In the meanwhile, she has gained some weight also because of her vacations. And now in this month also she has not had period since today. Please guide us