Hello Dr,I'm Getting Very Serious Pain In My Lower Abdomen

Hello dr,I'm getting very serious pain in my lower abdomen and in my back during periods and having heavy bleeding for 1st three days.and also my mensuration cycle is 22-25days.now I'm 35 yrs old and have a child.but such pain is having since I first have my periods at the age of14.i consult with many Drs but all of them have same opinion that it's natural.but due to heavy bleeding and 22days cycle,I'm getting weak and my hemoglobin is 10 and bp is 100/60.every month I'm taking 2 tab of mental spas.so plz help me how can I get solve all these problems.