I always have blocked nose after swimming. Am I allergic to the pool?

Pool & stuffy noses. While pools are sometimes sources of allergens like molds & pollen (especially if outdoors), most nasal congestion following swimming is likely due to irritants like chlorine, temperature changes from getting in & out of the water, or positional changes (as you swim, blood that normally pools in the legs while upright flows towards the head more easily and fills the nasal tissues).
Treatments exist. Check with your pcp or ent. Otc sinus rinses may help or prescription nasal steroid sprays may be of benefit. Avoid otc decongestant sprays.