Aww mama, ang stress mo talaga! First things first, if your gums are getting worse or the swelling is spreading, please message your OB right away or go to the clinic, kasi that tooth pain needs proper attention.
Listen, if your OB prescribed it, she wouldn't have given it if she thought it could harm baby, doctors know exactly which meds are safe in pregnancy, and that's literally their job. MILs mean well talaga, but they're not the ones who studied medicine for years, so try not to let the worry-talk sink in too much (easier said than done, I know!). Your OB knows your case better than anyone online.
Take the medicine as prescribed, and if anything feels off or the pain doesn't improve, tell your doctor right away. Teeth problems in pregnancy are actually super common, and getting it treated is better for baby than letting an infection spread. You've got this, mama!