Hello ma'am, it will be better if you consult a gynaecologist too.
Iron tablet in itself causes gastritis. By your history of gastritis, it's evident that you're experiencing it more. So if your Hb is good around 11 gm, you can take once daily Iron post meals 2 hrs. Also you can change to different iron brand formulations and see, if you're still nauseous.
With regards to the low lying placenta, at present we need to just follow up with the diagnosis.
For any more details or explanation, you can connect.
Next Steps
ANC profile - Investigations
Consultation
Health Tips
Plenty of oral fluids
Avoid Heavy work/ Heavy wt lifting
Avoid Vigorous sexual intercourse