Help! Has this (store-bought) yogurt gone bad?

[blumooned]

I just opened some organic cream top whole milk plain yogurt.  The best by date is Jan 20, 2011, so it is not expired.  Usually when I open a new yogurt, the top is relatively "dry" - there isn't any separation yet.  But, today, there was separation right when I opened it and it smelled a little off - not bad really, more like it was old, like it had been in the fridge for awhile, even though it hadn't.  I scraped off the cream top and threw it away because I thought that's the part that was off.  The rest of the yogurt seems to have a little bit of a strong smell and tastes really tangy.  It has been awhile since I bought this kind - I've been buying greek yogurt but the store was out yesterday.  Is this just the way this kind is?  Or has it gone bad?  I did taste it and haven't gotten sick or anything yet, but . . . should I keep it around or not?

Also, I already used some in a cheesecake I just made.  (I used it instead of sour cream.)  Will this be OK?  Even if the yogurt is bad, would cooking the cheesecake kill anything dangerous in the yogurt?  Please tell me my cheesecake is safe to eat - I've been hungry for cheesecake for so long & finally got around to making it!!

[swede]

I am sure it is just really ripe :)  I would use it without worry in baking, and if I could stand the taste, I would eat in uncooked. 

[blumooned]

Sorry - I forgot that I had posted this!  Thanks for the reply.

I ended up throwing out the stuff that was left.  I couldn't get "when in doubt, throw it out" out of my head!  We ate the cheesecake - it was fine & no one got sick or anything.  So, yeah, it would've been fine to bake with it, oh well.