Yup, probably fungus. I have seen this very frequently with bettas. Bettas are one of your scaless fish, and therefore their skin is very sensitive to abrasions (ie netting) and water quality. A betta who has been handled roughly, like at the LFS during jarring, and then kept in poor water will ususally develop fungus within 24hrs. Sometimes the disease progresses so fast, the fungus completely covers the entire body.
I don't even know if fungus is correct, the infection may even be more bacterial in nature. Either way, it can be very deadly.
One of the best ways I have found to handle it is with salt. I cut the dose of salt required for whatever volume the betta is in into a quarter, then keep the betta in that salt solution until the fungus goes away, changing the water daily. BettaMax is also a good medication to use to get rid of that fungus, as well as Maracide, Maracyn I & II tabs, and Rid Ich. Although with any of your medications you want to be careful for your betta may prove sensitive to it.
Just make sure you keep your betta's water drinkable clean at all times and you should be able to save your little buddy and keep him healthy for his lifespan.
~~Colesea