My engine shudders after shifting gears?

I have a 1997 Plymouth Neon bought new in 97 which i have since put 130,000 miles on.

i recently replaced the radiator on it because i noticed it was leaking antifreeze after a road trip (Moreno valley, CA to Las Vegas, ~500 miles in 1 day) and i discovered a crack in the radiator

I did the job myself and the new radiator works great, and while I'm not the most automotive savvy guy in the world, im pretty sure i didn't mess anything up, but its a tight fit so i had to take the battery compartment out to get the fans out.

the second time i turned the car on after replacing the radiator, the check engine light came on and the engine shuddered for a moment after it shifted into second gear.

it runs fine otherwise but the check engine light now stays on and every so often the engine will shudder after shifting into 2nd gear

any help would be appreciated, i just want to know if this is a serious problem, because i won't have any money for a serious repair for about a month.

I dont know if it matters but all fluid levels are normal, my spark plugs are good and as far as i know this is the first real problem ive had with this car (besides the radiator)