It's not just you! I used to joke that I never knew what legs were going to show up with me to ride - the good ones or the heavy ones!

It's good that you're increasing the number of days you ride, but be sure to vary your intensity. IMO you should have one really easy day every week - I mean really easy. An article in Bicycling described it as feeling like you're taking your bike for a walk. You shouldn't make every ride an intense one, and that's the tendency when you're trying to improve performance. But you'll be surprised how good and strong you'll feel more consistenly if you give yourself an easy-ride day every week. At least that's what has worked for me.