Some prenatal vitamins can cause constipation. If it continues, you might want to have your doc switch you to a different prescription.
I have the same problem. I use milk of magnesia, my dr told me it is safe is early pregnancy. And it is thr only thing that has worked for me.
In the early stages I was constantly constipated, only went once a week an felt so uncomfortable. Still not what I would call normal yet and I'm 35+4 weeks. It's fairly common as your baby is taking all the good stuff out before you go to the toilet.
Castor oil is a very bad idea as it can some times bring on labour, i toke it twice at tge end of my pregnancies to bring on labour and it worked, so i would not recomend that. Its very normal in early pregnancy to not be able to open your bowels, call your doc or midwife if still the same tomorrow.
I went days without #2 at the start of my pregnancy. If you ask your midwife she'll tell you what you can take for it :)
maybe it could be something you ate but i hear castor oil can give you a good bowel movement. maybe too good...i heard diarrhea but id drink lots of fluids. eating lots of green leafy veggies and metamucil help keep you regular as well