Might have an infected batch. Please help!

So, I brewed an American Brown about 5 weeks ago. I let it sit in primary for 3 weeks, prior to dropping 4 ounces of Chinook hops in with the beer.


Recipe: American Brown - 10G
Style: 10C-American Ale-American Brown Ale

Recipe Overview

Wort Volume Before Boil: 12.50 US gals
Wort Volume After Boil: 11.00 US gals
Volume Transferred: 11.00 US gals
Water Added: 0.00 US gals
Volume At Pitching: 11.00 US gals
Final Batch Volume: 10.00 US gals
Expected Pre-Boil Gravity: 1.051 SG
Expected OG: 1.058 SG
Expected FG: 1.014 SG
Expected ABV: 5.9 %
Expected ABW: 4.6 %
Expected IBU (using Tinseth): 35.6
Expected Color: 22.0 SRM
Apparent Attenuation: 75.0 %
Mash Efficiency: 75.0 %
Boil Duration: 60.0 mins
Fermentation Temperature: 68 degF

Fermentables
US Pale Ale Malt 20lb 0oz (84.4 %) In Mash/Steeped
UK Medium Crystal 2lb 0oz (8.4 %) In Mash/Steeped
UK Brown Malt 1lb 0oz (4.2 %) In Mash/Steeped
UK Chocolate Malt 11.00 oz (2.9 %) In Mash/Steeped

Hops
US Chinook (15.3 % alpha) 1.00 oz Bagged Pellet Hops used 60 Min From End
US Chinook (15.3 % alpha) 2.00 oz Bagged Pellet Hops used 15 Min From End
US Chinook (15.3 % alpha) 2.00 oz Bagged Pellet Hops used At turn off

US Chinook (15.3 % alpha) 4.00 oz Loose Whole Hops used Dry-Hopped

Other Ingredients
Whirlfloc Tablet 1.00 oz used In Boil

Yeast: DCL US-05 (formerly US-56) SafAle

Mash Schedule
Mash Type: Full Mash
Schedule Name:Single Step Infusion (66C/151F) w/Mash Out
Step: Rest at 151 degF for 60 mins
Step: Raise to and Mash out at 171 degF for 10 mins

So, the flavor is either sour or bitter. I can't decide. It hits the back of the tongue and it hits it hard. I don't know if this is a by-product of the hops for a relatively low gravity beer, or if I picked up a bug along the way.

If I let the beer sit out for a few minutes and warm up, the flavor is almost where I wanted it to be.

Another factor, which may help in diagnosing what's going on is the fact that the beer is slightly over-carbonated as well. It was on 35PSI for about 1/2 day longer than I wanted it to be.

Thanks in advance for you assistance!

I'm guessing it's carbonic acid causing the bitterness. The only reason I say that is because you mentioned it tastes better as it warms up. That leads me to believe it's losing CO2 and bitterness along with it.

Let it sit for a week and it'll probably be okay.