Why does PMS and pregnancy symptoms have to be so similar?

So its been a little over a year since my husband and I have tried to get pregnant. He was deployed. Well he got home almost a month ago and we have been going at it. I havent tracked ovulation, just been having fun. Well apparently I haven't been paying attention at all to my PMS symptoms since he has been gone because I cant remember if I have sore boobs. My boobs are sore and my lower back hurts. Im not suppose to start my period until the 18th. My periods are on the same day each month. I know its way too early to have pregnancy symptoms but my mind goes straight there and I wish it didn't. I've been having a little cramping, not bad at all but I do have endometriosis as well. Im promising myself that I will not test until I am late or even a few days late to just make sure. The agony!