If it is clogged just get a bucket of tank water, make sure its tank water, chlorinated water will kill your bacteria. Then swish the cartridge around in there for a little and it will get the big chunks out and help your flow along.I believe it can also mean your filter media is clogged.
Nope! I have the same filter, just means you are getting a lot of intake and the overflow is coming out that. If you don't like it, you can try closing the strainer (the little thing like 1/8 down from the main filter).