In my opinion and experience learning is an important factor i.e. the rats learn that the stimulus will become painful/unpleasant at the end and thus react by responding immediately. How often do you test your rats? Is this the only procedure for which the rats are handled i.e. can they anticipate that during every procedure there will be an unpleasant event?

These rats were tested once a week. Do you think that might influence their response on the filaments?

I just found out that the day/night rhythm during that period has been disturbed. This might cause stress in my animals which could result in mechanical hypersensitivity in control rats. Very unfortunate but a plausible explanation as I haven't observed the learning behavior you described before in my experiments in which I also tested weekly (although that was on female rats of lower weight)  

Weekly should be OK. Chronic stress could indeed be a factor causing hypersensitiviy although stress is more known to cause analgesia rather than hyperalgesia.

I would though expect that during the dark (awake) cycle the response time might be faster. Do your STZ rats develop the expected degree of allodynia?

Just another thought: von Frey measures allodynia thus in an ideal setting there should be no response from a normal animal. The filament to be used very much depends on the strain, line, gender experimental setting.. thus you should best establish which one to use for your specific situation as the largest one that does not provoke a response. It might also be good to use different pain assessments in parallel e.g. spontaneous pain, thermal allodynia, mechanical or thermal hyperalgesia..,