If you took a double dose of the pill on a single day and then accidentally didn't take it for two days, keep in mind that the contraceptive pill will not be as effective as usual if you miss some, though. You may notice a spot, sometimes bleeding, or a conception of pregnancy in your body. In the interest of safety, take your pills at the same time every day and, in case of doubt, use additional protection with prevention against sexually transmitted diseases like condoms for the next seven days. 

Having a small fibroid, about the size­ of a grape, can make it hard to poop and cause back pain, mainly whe­n you have your monthly period. Eating lots of foods with fiber and drinking wate­r helps with hard poops. You can also talk to your doctor about ways to treat the fibroid if it make­s you feel bad.

A normal endometrial thickness is usually between 3 to 4 mm, approximately 18 days after periods finished in women of reproductive age. It is advisable to contact a gynecologist who can evaluate you and recommend the best option for you.

One extremely important thing to do is to investigate the possible reasons for your periods' absence. Weight gain and feeling thirsty might indicate struggling with various issues, such as chemical imbalances or thyroid problems. There are multiple other excuses for your feelings. I hope you will go to a doctor to check what's what and get the right treatment.