Good game! 

Here is my feedback:

What did I like?

• The mechanics.
• UI/UX design. (it's good, but it needs improvements.)

What did I dislike?

• You die almost spontaneously if an enemy touches you.
• The gameplay. It wasn't fun (at least for me).
• The game is unpolished. 

To Fix

• The Collectables decrease your ammo, but they add power-ups/increase other abilities. That wasn't clear at first. 
• Collectables are imbalanced. Sometimes, they decrease a lot of ammo and increase other abilities by a tiny amount.
• You die, "You Died" displays, you click try again, main menu loads. That's not good.
• The in-game' texts aren't readable on a white surface. [1]  

Suggestions

• Add a health system.
• Add SFX. And possibly add music.

Endnotes

[1]

Pretty neat little idea! The main menu being so complete was a nice thing to see, being so rare and having so many nice details like the external links, but perhaps allocating more time to the in-game UI (making it readable even when on the already-white walls by outlining it with black, for example) or adding some sounds might have made for a more fleshed-out core experience. 

The most interesting part of your game was the constant need to switch between guns. Maybe emphasize that by adding more reason to do that. Add a timer that kills or damages you if you wait too long between pick-ups. Make the next gun more powerful if you miss less shots with the previous. Make pick-up combos, where you switch quickly enough between guns to then get a rarer one spawned in. 

Of course, emphasizing which gun you have would be easier with the UI change I mentioned earlier. Minimalistically speaking, you did a good job as well. : )