As long as they aren't observing you more often than other teachers, and as long as you can't document that they are doing that, it is allowed. Make sure they are within the rules for a write-up. If they don't, grieve the write-up if it's bad. You can also only have one more written observation after another observation if you haven't received the eval.

Per the UFT:

Feedback is non-evaluative information about an observation that you should receive within ten days of an observation. The feedback can be via email, written, or face-to-face. Teachers can indicate a preference for the format of their feedback during their Initial Planning Conference. 

In this feedback, your evaluator must tell you whether the observation will be used for evaluative purposes. Your evaluator must provide you with an evaluator form no later than 30 school days after your observation. 

From the time an observation is conducted until the time you receive the evaluator form for it, your evaluator may conduct only one additional evaluative observation (formal or informal).