Why does my beer taste like a skunk?!

Skunky beer is caused by light, it generates 3-methyl-2-butene-1-thiol, commonly called mercaptan.

You can get off flavors from temperature variation and bacterial infection but what you are describing is light skunked beer. Try to buy it from places that tend to move that brand has decent turnover.

Its OK to drink in that it wont hurt you physically. Personally, I would just chuck it.

Lightstruck, or "skunked", beer has been exposed to ultraviolet and visible light. The light causes riboflavin to react with and break down isohumulones, a molecule that contributes to the bitterness of the beer and is derived from the hops. The resulting molecule, 3-methylbut-2-ene-1-thiol, is very similar chemically and in odour to the musk-borne mercaptans that are a skunk's natural defences.

Innis and Gunn itself is already a 'skunky' beer. However, the reason for it being extra so is this. It was probably refridgerated and then warmed and then refridgerated again. I don't know why but doing that does gross things to beer