Hello
It is a very likely possibility that during aerobics stretching you might have strained the lower left as well as para spinal muscles.... do you feel tightness in your muscles on left side?
What is your height and weight?
Are you a working professional?
The problem can be simple muscle spasm or could be related to vertebra...hence proper history and symptoms are required.
Regards
Next Steps
Dr.Raviraj Francis
MPTh(Rehab),MPT(MSk).UK