Aww, ang common yan, sis! Maraming OBs talaga ang nag-prescribe ng calcium during pregnancy kasi importante sya for baby's bones and your health din. A few of my mum friends actually had the same experience, bago magswitch ng doctor, they were on one brand, tapos ang new OB nag-suggest ng iba pa.
Since 4 months ka na, best to ask your new OB directly kung bakit niya na-adjust, baka mas better ang formula na yan for you, o maybe dosage lang ang different. Doctors usually have their preferences based sa patient's needs, so wag mag-alala. Just take it as prescribed okay?