Hi
Sounds like your volume rocker is malfunctioning. If you've exposed it to a fall, or liquid damage, that could be the result. You can check to see if its software related, but trying a Cache Partition Wipe through the Recovery Mode; don't think that's it, but worth a try!
Google> "How to enter Recovery Mode on "your phone model here."
A trip to a repair facility might be in order. Changing the volume rocker should be relatively easy and inexpensive.
Hope this helps!
Chris