severe lower-back pain

i've had 3 pregnancies.1st 2 were miscarriages within 3 months,didnt have any nausea/vomitting.third was normal pregnancy with nausea,vommiting,lower back pain,oedema,etc.,delivered normally in the 40th week.my periods have always been regular before and after my pregnancies.my period were always 27-29 days cycle,after the last pregnancy its been 28-30 days,but regular.i've certain pms symptoms like oily skin,heavy breasts,water retention,feeling bit heavy overall,4-5 days prior to my periods and on the day of my period it goes away.i breastfed my son exclusively for 6 mnths,he's very much on solids now,just breastfeeds before sleeping,and sometimes i skip tht.now the problem is i've been having lower back pain since 4-5 months about 1-2 days prior to my period,along with all other pms symptoms.this month i got my period on 34th day,with severe back pain since a week before it,and still have it till date.usually it goes after period.had unprotected,withdrawal sex during fertile period