EC help, signed ff?

[cheako]

I'm attempting to improve the performance of my EC library, by migrating to this library. I know I'm not supposed to write my own and I do understand that ff does not offer consistent time.
https://gitlab.com/cheako/jtm-rs/-/commit/714b6d624c485882d3c64b7d6b3b0b8cb38ec92f#b24749917179fb5e3e613ed2a703fcdcc6cdf9da_291_226
Here I'm checking if the value module q is negative, my usual calculations are done module p. So my question is, can ff implement signed finite fields? I'm so confused about finite fields in general... I basically brute forced my way through implementing this using num-bigint-dig.