Why My Periods Have Become Longer/ Shorter After Miscarriage ?

I am a female, 27 yrs, with regular periods (26 to 32 day cycles) before my first pregnancy . I got preg first time in June 09, was later diagnosed with blighted ovum in Sept and pregnancy was medically terminated in Sept. with medicines only. After that I have had periods on 16 th Oct, 19 th Nov (35 day cycle), 23 rd Dec(35 days) which were longer cycles than usual. Now, on 14 th J anuary , I suddenly got my periods( just 23 days) which is short than my usual. Now, I had a doubt whether it may be a missed abortion and did an urine hcg test with the pregnancy strip (18 th Jan) and the result was negative. Now, can you please let me know why my periods have become longer/ shorter after miscarriage . Also , if I try to conceive now is there a chance that my egg quality may be poor. Any advice will be greatly appreciated. Thanks in advance Maria