Nice simple game :) Quite challenging once I got to the 3rd level!
I liked that you kept the scope small and focussed on doing the one thing well, which is a good idea for a jam.
I think a couple of things to think about/improve for next time:
>There wasn't much point in having the 1st and 2nd weapons as the 3rd was just objectively better each time. Always make sure every feature has a reuseable purpose in the game, otherwise its better to just not have that feature and save yourself the time making it :) Perhaps the first weapon could be an instant kill up close; or the third weapon could have a reload time or something like that.
>The level felt a bit cramped; I think if you had made each of the sprites smaller and spread out the level a bit there could have been more tactics for the player in terms of which walls to go behind, where to run to etc. I found myself just staying around the middle area most of the time.
>Change the way health is regathered. Having health regained as you killed enemies essentially means its a positive-feedback loop when you're doing well (which makes the game too easy) and a negative-feedback loop when you're not doing well (which makes the game too hard). I.e. if you suck at aiming you wont get any health AND there will be more enemies on screen trying to kill you, and if you are really good at aiming you will have heaps of health AND there won't be any enemies around cause you're killing them quickly. I think having health packs dropped randomly over time might achieve a similar way to keep the player alive, always hoping for that next health pack to drop soon.
>I get that you I was rewinding back in time, but I felt the first level was when i felt like the biggest badass, and when i got to the third level and my gun was slower I felt weaker. I think having a way to make the game get more interesting as the levels go along (like maybe on the first level you only have the 3rd gun, then on the 2nd level you get two weapons which do different things, on the third level you get a knife which instantly kills at close range; or something like that) is better than taking power away from the player as they get further (it did make the game more challenging, which was good, but it came at the expense of me feeling cool, which wasn't good).
Hope some of that feedback helps you if you do another similar game (and hopefully you do!). I hope that feedback doesn't seem too negative, I just like trying to give as detailed feedback as possible :) Well done on the game!