Please help as much as you can!

I'm on the Depo Provera injection, but it is close to the end of the 3 month cycle.
My partner and I use the withdrawal method as I've never had a problem with the injection.
Anyway, about 2 - 3 weeks ago I started a dose of antibiotics and we didn't think twice about using any more protection.
But over the last week I have slowly started experiencing early pregnancy symptoms.
Infact, I did a quiz and had 20 out of 20 symptoms. Yet low and behold, an home test and a blood test have both come back negative.
I already have a 3 year old daughter and when I was pregnant with her my blood test came back positive at 5 weeks.
Now, I know that usually most symptoms can be explained by PMS but I have never had any symptoms of PMS, so I am very confused.
I know my body and I know something isn't right but I'm just wondering if I am over reacting or if someone else has possibly been in the same type of situation?