Why did i suddenly get really slow???

um, coz you're old and fat??

Just kidding, but could the changes have somehow affected your efficiency or running form?

I didn't switch to minimalism per se, but I did go from fairly heavy trainers to lightweight trainers. My training was going very well, felt strong, but my first few races were shitty. Now I'm PRing like crazy in everything. Stick it out awhile longer and see what happens.

Probably just because this is the first race of the season--it's still early. I've found that even if I do a lot of miles, nothing really substitutes racing in terms of getting you in the best shape. My junior year of hs, my 3200 PR was 11:58 (I'm female), and even though I put in a lot of miles in the winter of my senior year, my first 3200m was 12:26. I was crushed, and wondered for a bit if my extra training was worth it, but within two weeks I ran 11:57, and then went on to run 11:36 for 3200m and 5:21 for 1600m (down from 5:38 the year before). Give it a few more racing weeks, and those times will come down.