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Her immune system is clearly compromised, and she is at risk for HIV d/t IV drug use. HIV testing should be offered, and I would also ask if she ever shares needles.

There could be other reasons, but this is my first instinct. Good luck!

P.S., for nursing Dx, I would go with risk-prone behavior & disturbed sleep pattern to start.

An STI and withdrawal would be medical diagnoses, not nursing diagnosis. What you need to ask yourself is what is going on with this patient that you can help with. What might an IV drug user/alcoholic be at risk for in addition to an STI? She's pregnant, so what might be happening with the baby? Once yu figure out your diagnoses, you plan your interventions. What are you going to do for this patient?

Do you have nursing diagnosis book, such as NANDA? That would be a good place to start.

An STI and withdrawal would be medical diagnoses, not nursing diagnosis. What you need to ask yourself is what is going on with this patient that you can help with.

Exactly. And this is the biggest mistake students make. You need to think like a nurse - she has disturbed sleep and night sweats. How might this affect her physical and mental health? How might this affect the pregnancy, or be affected by the pregnancy (or what comes AFTER the pregnancy, i.e. taking care of a newborn)?