Hi, I Am A Male At Age Of 42. Since

hi, i am a male at age of 42. since a week, i have been facing a problem in doing intercourse with my spouse. it becomes stiff at the beginning and when I start doing it at my knee position, it becomes softer.. this happened 2-3 times and i felt bad.
Also, through out the day since of a week, i feel that penis is softer and doesn't look the usual size. What could be the reason. I don't have diabetics, but cholesterol is around 225. I do walk 5-6 km's and some abs about 4 days a week.
Please advise me on this situation.