Don't worry too much. She will be fine, I am sure. You are on a huge learning curve, and are also sleep deprived.
I'm not denying that it is important to sterilise bottles and other things, but I am sure many of us know of children who come from unclean homes but seem to thrive. I have come across a few whose parents probably wouldn't have known what a steriliser was, never mind used one (think thick dirt on window sills and walls, grimy smelly carpet, greasy and mucky kitchen etc.).
You washed the bottles in hot, soapy water. That is also pretty good, so problems are hopefully unlikely. You have given yourself a bit of a scare (we've all done something like it), so you won't make that mistake again.
As others have said, constipation is not uncommon in bottle-fed babies. I bottle-fed all of my three. Occasionally they could seem constipated.
Ensure you are measuring out the powder accurately (I am sure you are), as a feed that is too strong can cause problems sometimes.
If you are still worried tomorrow then go to your GP. They will have heard it all before and most are very sympathetic. They should check your baby over and hopefully be able to reassure you that all is well.
I know how vulnerable you feel. I too had a pre-term baby. Not quite so pre-term as yours, as I had a 35 weeker. It was my third baby, so I wasn't a novice mum, but it did make me feel that she and I were more vulnerable at that point. She was soon thriving though.