I had a similiar problem. Mine started very hard last time out, and then after many hours of riding that same day, the temperature light came on. The coolant bottle was way low and when I took the cover off, what fluid was left in there quickly disappeared.
Tonight, I filled it back up with antifreeze and started it up. After it warmed up, I checked the compression and had (left to right) 120, 120, 90. I put the plugs back in, started it up and while it was running I checked it over. That is when I noticed a very slight leak coming from under the head gasket on the cylinder at 90psi. I could wipe it off with a cloth and then it would reappear again, and very slowly. Every time I would wipe it dry, it would get wet there again.
So you might want to start it up and look all around the cylinder heads for just a little bit of wetness, right where the head meets the jug. I couldn't see it with a flashlight at night on the trail, but it just barely showed up in daylight today.
Not a big deal to pull the heads off. 6 bolts per head and disconnect the coolant rail - another 6 bolts and a couple hoses.