I have a problem with my bathtub water flow. I have a faucet with one knob that controls both the hot and cold water flow. I can pull it out with and have full water flow only if it is dispensing both cold and hot evenly. If I try to turn it to give me more hot or more cold the water flow lessens to almost a drizzle. This problem just started and I am not sure what the problem could be. I am not well educated when it comes to this type of problem. Could someone give me advice on how to fix this inexpensively without calling in a plumber?