My Oktoberfest still fermenting after 4 weeks?

[NoobBrewer]

After my Oktoberfest fermented for 10 days in the primary I did what I usually do, Raked it into a BB. There it sat for almost 3 weeks. I went to Cold crash it today and noticed about a 1/2 inch thick layer of suds floating on top of the brew. I took a look closer and saw small bubbles climbing the walls if the BB forming the suds. I've done about 15 batches or so by now and this is the first time this has happened. Could it still be fermenting or could it be an infection?? ( no bubbles have come out of the airlock for 3 weeks now) also this head floating on the bew wasnt there last week or the week before.

 

[Wendy1971]

sounds like its fermenting but it shouldnt be if its cold.