Yup, it could be tension or maybe your embouchure is tiring very quickly due to lack of playing, but that might be dependent on how long you've been playing - if you're a relatively new starter it could take longer to 'find' your embouchure again, which could be resulting in the aforementioned tension.
'Something Else' is indeed a great album.... to play along with! There's a couple of good mid-pace tracks (Autumn Leaves and a fairly straight laid-back blues) that are quite easy to get along with.
Cheers, Bob