If you lowered it without getting an alignment, I would expect unusual tire wear. However, depending on the ability of the alignment shop you took it to, they may have set it incorrectly or not at all. Many of us have come across shops who set toe only and call it aligned since so many late model vehicles cannot set caster and camber. Without a spec sheet, we're just guessing. Of course, if you take it in to have it redone, you can probably get a before and after reading to know what was wrong.

It is also possible they set things, but forgot to tighten the upper arm bushings and they took a set with too much negative camber as you pulled away.

I'd ask for 2005 Mustang GT specs, which are -.75* camber +7* degrees caster and .15* toe in. They won't be able to hit them, but it gives them a target to shoot for and when they fail, then they will have to talk to you about it and you can decide to approve the results with may be a compromise between 0 to -.5* camber and as much positive caster as they can get.

Don't take this the wrong way, but based on the question you are asking, and the fact you commented you can "see" some negative camber, I would guess one or both, you have a lot of negative camber, or your toe in is way off. The negative camber by itself does have a few very nice upsides, but also a few downsides, like gross tire wear for a daily driver. If you want to be more precise with your next alignment, which should be next before making any other changes/fixes, throw a few bags of ballast/sand/etc in the drivers seat when aligning.