Treadmill is killing me

Drewso

So I have been doing my running on a treadmill lately because of the weather here. Anyways, I find it so much harder to run on that thing lately. I dont get it. I used to love it and be on it for an hour or so. Now it mentally and physically drains me. I dont know why! lol. Its killing me! I did gain some muscle since last year, but I didnt think it would affect me running on the treadmill this much. I dread it now. Think I need a break from the mill or just some motivation? Different cardio?