Difference between early pregnancy cramping and pms?

TTC4BFP
Hi ladies! 
I'm four days past ovulation and I have a feeling that this month is my month! (Well I'm hopeful) 
I'm having a lot more lower abdomen cramping than usual but I'm wondering if it's also pms. I don't usually feel a constant dull cramp especially so far before my period (13 days away)
Ladies that have been pregnant before, How can you tell if it's implant ion / early pregnancy or just pms?!!