I had the same thing happen to me when I first started cycling. I was overweight (still am) and on meds for BP and cholesterol. I was doing great. 15-20 miles during the week and 30-40 on weekends. Then, like a switch was flipped, I felt like all my strength left my legs. The guys that I would have to wait on were suddenly waiting on me. Turns out it was the statin cholesterol meds. I could skip it one day and I was back. Take it again and no strength. I talked it over with my doctor and switched to a prescription fish oil tablet.