Why?!?

So I gave birth 4 months ago and my husband and I have been trying for #2. Well the past two weeks I've had back pain, cramps, sore breasts, been really tired and had headaches. I haven't had my 1st period since giving birth yet and I'm not breastfeeding.

I've taken so many tests and I thought two of them had a faint line. So I had a blood test to be sure and it was negative. Why would I be feeling this way?? I have PCOS and Endo but those wouldn't cause these symptoms would it? Are blood tests ever wrong? I just don't know what to do?!? Any advice would be great!