Here's what I understand of it:
After you ovulate your body is preparing itself for implantation and your progesterone levels stay quite high. Progesterone can be a cause PMS symptoms; so you can have PMS for up to two weeks before your period. If you don't conceive the progesterone levels drop, and if they drop, that's when you get your period. If you get pregnant the progesterone levels stay raised; and HCG rises too.
Other hormones also rise and fall during your natural cycle and it's believed it's the fluctuating levels which causes PMS symptoms like sore boobs, irritability, changes in appetite and even nausea for some women.
Unfortunately, PMS and first signs of pregnancy are extremely similar, if not the same, so it can be near impossible to tell them apart. Some women do notice slight changes and get an inkling that it's not just average PMS... But the only way to really know is when you're late and/or you POAS. 
They say that it's impossible to get pregnancy symptoms until implantation because your body hasn't started producing pregnancy hormones, and effectively doesn't know it's pregnant yet. And it can take up to 10 days to implant as the egg has to travel down from the Fallopian tube when it's fertilised.
My first pregnancy symptoms were cramping on and off (which I also get before my period sometimes, so thought nothing of it, except it started earlier and it lasted longer until it became constant. It was the type of thing where I had a moment of thinking "oh, I wonder..." and then brushed it off) and also burping a lot and getting trapped burps; which started happening around the time I implanted. This isn't something normal for me so it was a sure sign, but I didn't know that at the time and wondered if it was a stress symptom or something. Again, I wasn't sure until doing a test...
If you're in tune with your body you might notice subtle changes to your norm; but on the other hand PMS can vary month to month and if TTC is on your mind a lot there can be a psychological factor getting in the way! You will second-guess every twinge, gag reflex, sensation, smell, taste etc...
My best advice. Find distractions in the TWW... Lots of distractions! 
Best of luck! Fingers crossed for you