Whew, my Hot Tub (Spa) smells!
The smell you are smelling is most likely Chloramines, which is Chlorine and Ammonia combined. Ammonia enters the water usually as bather waste. Also Plant decomposition and fertilizer runoff can also introduce Ammonia. The cause of this is too little chlorine or bromine present while bathing in spa or Hot Tub.
The best way to get rid of the smell is to do a Drain and Fill of your Spa. You can also try to add lots of chlorine to burn off the chloramines (or Shock Treatment for Bromine treated spas). If the spa water is not in too bad of condition, then this is fine. I recommend a Drain and Fill, since a Spa is a small body of water, you don’t want to be bathing in a chemical cocktail.