I too would worry about the social distancing, esp as you are now pregnant and more vulnerable!
Have you been drinking/been out with this group since you started BF? Couldnt you just say you need to BF and dont want to drink 'too much'? Then substitute drinks?
If you use the excuse of antibiotics, they are called 'metronidazole' with the brand name of 'flagyl'. They are used for certain dental and certain vaginal infections! They can give you a metallic taste in the mouth, make you feel nauseous and not very nice to have to take!
Many years ago I had them. A day later, after finishing the course, I had some wine with a girlfriend. After 2, small glasses, I was literally crawling around her floor. I had no idea why I felt like I'd downed several bottles. The term 'blind drunk' rung true. I could hear her, but couldnt open my eyes to see her. It was the most awful experience I've had! However, I woke up perky the next day, as only had 2 drinks!!!
If the hot tub night is a while away, I'd plan beforehand. The week leading up to the night, could you get in touch for a friendly chat, and throw in the fact that you've had a vag infection and on these awful antibiotics? Obvioulsy depends how close you are to these girls, whether you feel comfortable lying and talking about a vag infection, but its an option. Keep us posted with what you decide.