LittleBigMan nailed it. Your muscles don't develop while you are using them, but while you are resting after you use them. Exercise tears muscle fibers, and it takes around 48 hours for this damage to repair, and the result of this repair is increased strength. So exercising every day does not allow development - just exhaustion and delayed development.

Taking this in mind, you could even try NOT riding two days in a row for a couple of weeks, maybe three. Maybe ride on Monday, Wednesday, and Friday for two or three weeks, and see if that helps. At least give yourself a few days off here and there.

Maybe find a couple of books on bicycle race training - I know you're just commuting, but you'll find the basic information about training and exercise in there. I am a distance runner and marathoner, and I found that I did the same thing when I got my bike. I jumped on it and didn't treat it like exercise - just like transportation and something to ride like mad, and I wound up injuring myself within a month - tenidinitis - from overdoing it.

Or just ignore this.