ceiling fan rotate slowly in my bedroom only
You need to purchase a cheap volt meter (make sure it can measure AC) at any hardware or large general store.

At the ceiling fan electrical box you need to measure the voltage.

Here is a good tutorial to get you started.

Once you measure your voltage (depending on where in the world you are) check and see if it is close to what should be reaching your outlet/fan.

I am assuming you will find that it is lower, if you do at this point you are going to have to troubleshoot that circuit. There are a few ways to do this with different forms of technology but the cheapest by far is the old fashioned method of tracing the wiring.

If you can get into the attic or crawl space do your best to trace the physical circuit throughout your house. You want to see what other devices are on this circuit. Are there any dimmer switches or other appliances?

My guess is that you will find something else on the circuit draining voltage. Find this device, remove it from the circuit, and your fan should operate normally!