What does it mean when a client is experiencing more pessamistic behavior after being on anti depressants?

See a doctor. The patient needs to be seen by a doctor. Some antidepressants can increase the risk of suicidal behavior. It's possible that the medication simply is not working, and the patient may need to be placed on a different medication. It's also possible that the medication was recentrly started, and has not had enough time for the medication to work. In any case, the patient needs to be seen.
Vary. When people appear to get more symptomatic on antidepressants, possibilities are, your depression is worsening and dosage is not optimal. Possible underlying bipolar d/o with worsening of depression, you may have an emerging physical problem like hypothyroid, making depression worse. Your psychiatrist need to asses you and carefully monitor your target symptoms as you progress in treatment.