I'm composing a song, and I made it so it goes 3/4 for 1 bar then 4/4 to kinda create a jerky-weird feeling in the song. But I have no idea what timing that would even be named as? Any help is much appreciated :)
As alephzero comments, it could be written out in 7/4, but a couple of criteria crop up. What will the next few bars be? If the 1st beat of the 3/4 bar is as emphasised as the 1st beat of the 4/4 bar, then it needs to be as you wrote - 7/4 is usually split, and the second 'half' won't be quite as emphasised as starting a new bar, in whatever time sig.
The label would probably be 'irregular time sig.'.
Another way to do it is to put expression marks over certain notes, but keep a regular time sig. I seem to remember something like that in Stravinki's Rite of Spring.
To further compound (confound?) the issue, it needs to be clarified that the crotchet b.p.m. is the same for all the bars. It's also possible to have a situation where each bar is the same length of time instead, although the first option is the usual one.
The formal term for this is mixed meter. If the piece strictly goes back and forth between only two meters, it could be clearly described as alternating mixed meter.
Another great (and very famous!) example of alternating mixed meter is the song America from West Side Story, which alternates every measure between 6/8 and 3/4 time.