What Causes Sudden Weight Gain With Irregular Periods At The Age Of 41?

I'm 41 yrs old have had 5 children also had my tubes tied twice. I have weighed no more then 86 pounds my entire life except while pregnant. My stomach is growing, AT WHAT I CONSIDER TO BE A REDICULOUSLY FAST PACE. MY Breast are also sore and growing. I do not have regular periods can't remember my last "normal"cycle. I also suffer from MS and take daily pain mess for that.
My concern is why I'm getting fat for the first time in 41 yrs. What could be wrong?