Caution is advised with this combination as it does increase the risk of serotonin syndrome and neuroleptic malignant syndrome, however it is up to the doctor's discretion. You also have to take into consideration how often you are taking the medication. The max dosage of flexeril is 30mg within a 24 hour period. If you're taking 5mg three times a day, you're only taking 15mg in a 24 hour period. An interaction with the paxil is unlikely.
It was never mentioned to you simply because serotonin syndrome is not a frequent occurrence with this combination, although it does happen. If you're still concerned, however, you should speak with your physician and ask him to change your medication. There are other centrally acting muscle relaxants that do not have this interaction with paxil, such as robaxin, skelaxin or soma.