What Causes Severe Pain In The Lower Back And Pelvic Area?

Dear Doctor,
I am a 27 years old female married for 7.5 years. I have a 4 years old daughter, before conceiving her I had Polycystic Ovaries Syndrome and was on Glucophage when I conceived her but stopped taking it when I found out I was pregnant. There were no infertility issues it was just that I got married at the age of 19 and didn’t wanted to be a ‘mommy’ so soon that is why we delayed having a baby.
My daughter was delivered through C-Section because the CTG was not reassuring. Since delivery I have had fairly normal menstrual cycle. My periods last for 3days and I have a 28 to 30 days cycle; for the past 3 months they come exactly 5days early. I had periods on the 20th of June; I was anticipating having them on the 15th of July, around the 17th of July I started having lower back pain, cramps in abdomen. Initially I thought they are the regular menstrual
> cramps but when I got 2 days over due I thought our birth control method might have failed, we use the withdrawal method which is obviously not 100% reliable but it has worked for us for the past 7years. I was having nausea, feeling dizzy, and feeling hungry more than normal. Although in my first pregnancy I started pregnancy symptoms after I was 9 weeks pregnant. On the 23rd of July I did a home pregnancy test which came out negative. On the
> 25th of July I got beta HCG done which also came out negative but
symptoms
> were there. I went to see a gynecologist on the 30th of July she
> did an ultrasound and said I have a 12mm cystic area in the uterine
cavity,
> it may be a gestational sac of irregular routine scan and I should see
her
> after a week and repeat beta HCG. No cysts were seen in my ovaries. She
> said I might have conceived late but she can not definitely say if I am
> pregnant.
> On the 31st of July I had vaginal bleeding; I called the gynecologist she
> asked me to wait till the next day (which was 1st August) and come
to
> her hospital and have a DNC. Initially the bleeding was not much but I
had
> severe lower back pain and pain in my pelvic area and I got really
worried
> I might be bleeding internally since the doctor told me I will have a
DNC.
> But on 1st August I realized that these are my regular periods.
> My question is in your expert opinion will you be able to tell me
> 1. What can that sac be?
> 2. Something to worry about?
> 3. We want to have a baby so may we start trying from this month or wait
> for sometime and repeat the ultrasound to check on the sac?
Thank you very much for reading my problem will be waiting for your
reply.
> Hina Ali