i'm 6months pregnant & I took pentrexyl,what can happen??

According to the FDA, common antibiotics that are generally considered safe during pregnancy include penicillins (such as amoxicillin and ampicillin). I believe Pentrexyl is a type of ampicillin (it is not available/approved for use in the U.S.).

If you are still concerned, you can speak with your doctor, who will need more information, as the answer depends on so many factors:
When did you take this antibiotic, how much did you take (dosage, frequency), what did you take it for? Did your prescribing doctor know you were pregnant? But, please know when a drug is "considered safe during pregnancy", that when used correctly, it means that no studies have shown any harm to the fetus. You can read more about how drugs are categorized in pregnancy at this discussion (related to allergy medication): https://www.empowher.com/community/ask/are-allergy-medicines-ok-take-while-pregnant

Most of the time when doctors prescribe medication (to a woman known to be pregnant) it is because the benefits to the mom of taking the drug (to cure an infection, that could be a risk in itself to the baby) outweighs the possible risks to the baby.

I hope this helps, and let us know if you have any additional questions. Most importantly, congratulations on your pregnancy, and has your infection been properly treated?