Welcome.
Impossible to know without fully testing your water. A foul smell is never good.
To prevent water/tub problems you need to be able to test your water (yourself) regularly and properly sanitize your system.
You should develop a sanitation plan for your tub.
Get a good test kit that can test for pH, Free Chlorine FC, Combined Chlorine CC, Cyanuric Acid CYA, Total Alkalynity TA, Calcium Hardenss CH and learn to use it.
List some information about your tub and system so others can help you. Specifics include tub brand, # of gallons, tub surface type, current sanitation practices, how long between drain and refills.
This info can be listed in your signature if you have future issues or questions.
This site has all the information you need to safely manage your tub so head over to this section....
Spas and Hot Tubs
Many good people who enjoy hot tubs and spas on the site as well, more than willing to help you out along the way too.