Yes, this is completely normal. It's nice for you that you have such a regular schedule! Most women have to worry about fluctuating month by month. This probably happened since you got your tubes tied because your hormones are balanced.
A 21 day cycle is normal for some women. If this is a sudden change in your cycle it could also be normal. How old are you? I assume if you had your tubes clipped 15 years ago you are at least in your late 30s? If it is a sudden change in your cycle it could be perimenopause (usually starts in the late 30s). Or it could be just normal for you.
There may be nothing wrong. But you are definitely bordering on things not being normal. Mine were every 21 days and my gyn is very concerned about it though we haven't been able to figure out what is wrong (we do know I have hormone imbalances from a pituitary tumor).
They will probably want to check you for hyperthyroid and some other things. I'd get it checked out.