No period 4 months pp

My son turned 4 months this week and I still haven't had a period! I've been back on birth control since 2 weeks pp. I was told my period should have returned to normal because I'm back on contraception but they haven't. Every month when their due I get all the signs and symptoms just no period! I have cramps and hot flushes etc... I just don't bleed! I'm not pregnant I have done 3 tests just to make 100 percent sure! Anyone else experienced this? My doctor keeps saying it's fine it's normal... but then other doctors I know etc are saying to get it checked :/

Update:: I forgot to say that I am not breastfeeding I'm bottle feeding :(