Pregnant or not pregnant? :/

Tara
So i had sex ( no protection)  with my boyfriend (now ex) on 26th July. 
I took a pregnancy test two days later but came back negative so I never thought anything if it and brushed it off. 
But just the last week or so I have been getting cramping  like someone is stretching and pulling my ovaries and it last pretty much all day. 
I'm getting heartburn at night which I haven't had in years. :/ 
I have checked my glow calendar and it tells me I'm in the middle of ovulating. 
The problem is my periods are all over the place and I don't exactly know when my next period  is due to being messed up all the time.
The reason I'm asking is because they say take a pregnancy test a few days before your next period . 
What do you think I should do my Glow friends ?