Yes, as Mak said, fiber and cable are two physically different technologies. Do you get your TV and internet from the same provider? If yes, then you are on cable, and your problem could be attenuation on the line.... Additionally, when you have cable, you are essentially on a node that is shared by all of your neighbors who have the same service. If there are a few people hogging all of the bandwidth, others will suffer. At my old apartment, I had "6.0 Meg cable"... it would be less than 1.5 Meg speeds during peak times in the day, and the full 6.0 Megs at night due to the sharing on the Node....
If you truly have fiber, it would be a fiber optic cable coming into your home, and you would have a gateway of some sort where the signal is moved from optical to the copper.
Might you be able to provide a picture of the device where the fiber terminates at your residence, if you truly have fiber?
Also...... I am thinking this is the scenario..... your residence is fed by copper coaxial cable from the Node, but at the node, it jumps onto fiber to wherever they distribute the services from.... DSL & cable generally operate in this same way. Just because the NODE if fed by fiber (because it is a backbone, and fiber has a MUCH larger bandwidth than copper), you can't say "I have a fiber connection".... If you have a fiber connection to the internet, it is LITERALLY a glass or plastic fiberoptic cable that comes DIRECTLY to your home.
Below, you will see a Fiber NID for Verizon's FIOS service.... You can see the green cable on the upper left is the fiber coming into the NID, and on the mid right, you can see where the data service, POTS service, and hiding in the background is a Coax cable for TV service....