What Causes Pressure In Uterus And Low Hormonal Levels Leading To Miscarriage?

Hello, I ve been trying to find out about my problem but without success.. Now my last three pregnancies, I start feeling the uterus pressure and pain even 14 days before my expected mentrual cycle. I already know that I am pregnant then. It so painful and discomforting that I need bedrest. When it s time to do the urine and blood test, it will show low hormonal levels. And I get a miscarriage from one day to a week after the test. Before these cases I had also complications in my pregnancies, with partially detached placenta and bleedings, which resulted me in having bedrest for a few months. Now I feel the pains again and on bedrest. My expected mentrual day is about 5 days away. I hope someone can finally answer... It is a very sad and depressing situation, but praise be to God for all His other blessings. (P.s.: I hope I explained it clearly) Thank you for your attention. Kind regards