Electric - power drain - dim to dead

Im driving a 1994 plymouth acclaim with roughly 200k and a 2.5L engine. Driving home after coming from the shop, I was running into a power drain. Head lamps, internal lights, and wipers were becoming slower/dimmer gradually until it finally became unsafe to drive and I had to take it off the road. Several minutes after stopping I tried to turn it over, and it was completely dead. While waiting for the tow, I desided to try it again. It started up again just fine, and seems to run fine. I dont feel confident to take it on the road until I get it fixed. The battery is new(2 weeks old), and appears to have a good charge(verified battery I purchased was proper for my make/model). I think its an issue with the alternator, from what I know about auto electrical, but I want to make sure before I take it back to the shop. I removed a security system a month back(ensured all wires were re-attached properly, no lose connections or open wires), and that was the last time there have been any changes to the electrical. The mechanic I work with is trustworthy, I know he hadn't made any changes. Any ideas or confirmation of the problem would be great.