Lost Weight-- do I need a new HRM band?

So I lost weight since I started. When I did p90x ab ripper x video, I would usually burn about 90-95 calories, consistently. However, I tried it today, and the band felt as if it was turning when I was doing some moves, and my HRM monitor only calculated 45 calories even though I threw in an extra set of leg climbs in addition to the usual video. Is it possible my band doesn't fit anymore?

I have a band that says it is from M-XL. I don't know where my tape measure went, so I'm not sure what my inch measurement is.

Do I need a new band, or is there a way to tighten it beyond the max? (I have the thing pulled all the way, so that it is the smallest it can get.)