The reason is because you are trying to work away from catabolism (due to the lack of test in your body) and restart your body's natural test production. Not a good idea to run a thermo.
Either cut at the end of your cycle, or wait until you know you have fully recovered (blood panels) before you start any thermos.
People say Clen is ok because it exhibits some anti-catabolic effects, but i think thats bullshit and why would you even take the chance.
Best thing to do is wait IMO.