heading for overcarbonation?

I'm working on my first batch of beer and I'd like some reassurance that I'm not making bottle bombs! I bottled 8 days ago and haven't had any trouble, but I was really shocked at the huge amount of CO2 blown off of a bottle I just opened.

details of the brew:
Boiled 3.3 lbs dark LME in 1 gallon of water for an hour. Added about another 1 gallon. Pitched a 6 oz packet of Cooper's yeast. Primary fermention was largely complete 36 hrs at 75-70 degrees fahrenheit. Sat in the primary for 11 days in total. I checked the FG which rang in at 1.011. I added 1/3 cup of white sugar and bottled. I was a bit surprised to have only 1.5 gallons of beer.

The very specific question on my mind is: does beer continue to build carbonation after the first week in the bottle?

Yes - the beer will continue to carbonate after the first week. It usually takes 3 weeks to reach a good carbonation level (with the appropriate amount of priming sugar).

What temp did you store the bottles at for carbonation? - it will move along faster at warmer temps.

Did you chill the beer before you opened it?

I'm not sure how much 1/3 of a cup of sugar weighs, but for a 1.5 gallon batch, you only need 1.5 - 2 oz to get a lot of carbonation (3 - 3.5 volumes). If you're over 2 oz, you might be inline for some bombs (though I don't really know how many volumes of CO2 you need to have to reach explosion)

See The Beer Recipator - Carbonation

Most people use corn sugar for bottling. Going by the charts on the linked page they suggest 2/3 cup white sugar for a 5 gallon batch. That means your 1/3 cup is about double what is suggested. Very likely you will end up will bottle bombs .

Miscellaneous Brewing Charts

Any suggestions to avoid exploding bottles with this current batch? open the bottles, perhaps pouring some off?