Im on the progesterone only pill and im having pregnancy symptoms but have had a negative result, could i still be pregnant?

I am 20 years old and on the progesterone only pill. I used to be on cerezette (was on it for about 8 months) but had to change to a different mini pill a month and a half ago as mine isn't being produced atm. I was told that i could go straight onto it and that there should be no complications. The only difference is this pill only has a 3 hour window. The week before last (about 10 days ago) i took the pill late and had sex, and recently have been having early pregnancy symtoms (symptoms)- nausea throughout most of the day, lower back ache, stomach twinges. I thought that i might just be worrying but then the day befre yesterday i had some brown discharge/blood when i wiped after going to the toilet. This has happened quite a few times over the past two days now. This has made me worry as since going on the mini pill around 8 months ago i have never had any bleeding what so ever. I took a pregnancy test today and it was negative but im not sure if iv taken it too early. Im not sure when to take another one as i cant go off missed periods as i dont have them! Do you think i am pregnant? Any advice would be much appreciated. Thanks x