More carbonation in last few batches... Why?

So for the last few batches I have made I have noticed a marked rise in the level of carbonation present in the beers I have been producing - I was wondering what this might be due to? I'm not making recipes that have especially great amount of sugar in them and have been using a similar yeast the past three times. I am brewing in plastic TrueBrew buckets and all I can come up with is that somehow, maybe, there is some sugar or other fermentables probably still stuck in a little scratch or something on the inside of the barrels... Is this a possibility that might be affecting the carbonation?

All I'm saying is, the last few beers have had a lot of bubbles floating around in them and are having a hard time with head retention as well.

Perhaps its the temperature at which I'm storing the bottles? Is 55-60 degrees Fahrenheit too warm?

Any advice or input is appreciated.

What temp did you bottle at? Are you making ale or lager?

It depends on how you've been calculating your priming sugar amount and if you've been considering temperature of the beer at bottling time. If you're adding priming sugar by volume (e.g., 3/4 cup), it could be because your recent beer batches have been bottled in cooler temperatures, which means that there was more CO2 in solution at the time of bottling, thereby requiring less priming sugar (which should be quantified by weight, not volume).

This might help: The Beer Recipator - Carbonation

Bottling temp could be why. If you had a very cool secondary, your beer is going to have more CO2 already in it, from fermentation, and thus your priming sugar will give you more CO2 volumes than you're used to.

slight differences in final volume, and measuring sugar by volume instead of weight can also create unwanted variables that change carbonation.

55-60 is acutally a little on the cool side for bottle conditioning. 65-70 for 3 weeks, then you can keep it cooler for storage.

Also make sure you are chilling the beer 36-48 hours before you open it. Make the beer cold, then it can absorb the CO2 from the headspace. 4 hours in the fridge is insufficent.

As for unintentional fermentables in your plastic buckets, you'd have to have quite a lot of hidden fermentables in scratches to make that the cause. That would almost certainly cause a bacteria problem before it would cause anything else of this sort. (Don't worry about that either though!!)

How much sugar do you add?
How many days from brew to bottle?

Bottling too soon can be an issue, as fermentation of sugars in the wort will continue in addition to the fermentation of that added priming sugar.

Well, I almost always add the standard 3/4 cup of priming sugar and have been the past few recipes and I suppose my bottling temperature was probably somewhere around high to mid 50s each time. The past few brews have all been ales.

From what I've garnered here I suppose I'm going to try a little less weight in terms of priming sugar given the relative temps of my secondary fermentation and my bottling temp. Hopefully that will at least give me a heads up if it's a possible solution.

I usually give it a week and a half in primary and 2 1/2 weeks in bottle at about 62-65 degrees. If I rack to secondary, it's usually about another week and a half.

...Don't get me wrong - the beers haven't been bad. Actually, one of them got me 4th place in a local homebrewing comp. I'm just a little worried about this recurring more and more without resolve.