Can heat affect how difficult running is?

I am training for Cross Country. I just started my training back up again after my AAU basketball season ended. I just went outside and usually I can run 4+ miles no problem. However, I only ran about 2 - 3 miles just now and I found it difficult. I was shaky and thirsty, probably because it was about 90 degrees. My question however is did I feel more tired than usual because it is hot out, or because I haven't run like this in a while, even though I've been keeping my cardio going with basketball and other training.