This is good question. It's a verbal. The 'm is obviously short for 'am', which is the auxillary verb in this example. The purpose of an auxillary verb is to help another verb, whose form will be governed by the auxillary verb. In this example, it demands a participle (which is a verbal).
Now, a verbal is either a noun or an adjective. So, which is it? I am having trouble getting my teeth into that bit, but if your question contains one of the correct answers, then we can deduce that it is a noun - a gerund. This is supported by other similar examples:
I am going mad.
I am swimming the channel.
You see, gerunds (although classified as nouns) can take an object.
Short answer (and I willing to be corrected): It's a noun (a verbal gerund).
