Possible Infection?

I brewed a nut brown ale February 16th (partial mash) left it in the carboy 4 weeks then bottled. After bottled it had a good malty flavor with a roasted finish (possibly a bit strong for my taste, but still good). I just opened another one yesterday (and again today) for the first time in a month and it was completely different.

Had a sweet aroma, maybe some plum, and sort of smelled hot. There was no sour/vinegar aroma that I would expect from an infection.

The flavor was tart, maybe sour. It was definitely hot. There was no roasted finish. The beer poured with a good head and had good, actually great head retention but a lot more carbonated than I remember.

I used Wyeast (1272) American Ale II which is a little fruity, but shouldn't overwhelm the finish. I fermented at 68 degrees with no unusual smells and stored the bottles at b/w 66 to 68 degrees.

So: is 6 months in the bottle too long for a Nut Brown Ale? (pretty sure the answer there is no).
Do I have an infection?
If there is an infection, could it have taken place in my primary fermenter and taken almost 6 months to come through?

Luckily I have already drunk all but 5 (now 3) but this is still quite bothersome.

Thanks for your help.

I believe an infection would have given you gushers. Is there any kind of residue in the bottle? That could mean there is an infection. If anything I would think it would mellow over time. Maybe too much priming sugar? Maybe the beer wasn't finished before you racked. But either way I can't help much more then that. Hey, at least you got to drink some beer.

No residue in the bottle (just yeast) and it definitely wasn't gushing. I waited 4 weeks before bottling, skipped secondary because I was lazy.

I primed to style for an English brown, and at the low end of the range.

Could the yeast (American Ale II) have just taken over?