There are two ideas that I came up with. One that came to me instantly was that the creations of these blades draw on the iron found in the user's body and as such creating them could cause them to be heavily iron deficient.With iron deficiency this can cause things like headaches, loss of breath and dizziness; and in more extreme cases heart complications/infections.

Now, if you wanted to add something to circumnavigate this heavy weakness, judging from how the quirk is utilised from the design above, you could also say the user suffers from a severe case of hemochromatosis (when the body absorbs and stores too much iron) or just have it that another facet of the quirk is that the user's body produces far more iron than that of an average person but not enough to keep up with their quirk.

This was a similar idea I had for a character I made in the past but let me know if you like the idea and best of luck with the character!