4w 3d heavy aching feeling in back/pelvis??

Hi everyone,
I am 4w3d. This is my second pregnancy but will be my first baby. 
I have been feeling a heavy and achy feeling in my lower back/vaginal area the past 2 days, kind of like what I get just before AF starts - is this normal? I have had no cramping or anything just this uncomfortable dull ache...
I'm a bit worried because I really don't feel pregnant in the slightest.
Thanks in advance x