Your doctor is right to continue. Going from 12.8 to 26.8 means you have only just come out of deficiency - the target is 30 to 60, and in IBS absorption is poor, so levels climb slowly and fall back fast if you stop early. Weekly 60,000 IU for another 6 to 8 weeks and then a maintenance dose is standard and safe, provided your
calcium is normal.
Vitamin D toxicity does not happen at these doses; it needs months of very high intake. Take the capsule after a meal with fat for better absorption.
Next Steps
Complete the course your doctor advised, keep Shelcal with it, and repeat
vitamin D and serum
calcium after 3 months. Add 20 minutes of sunlight and treat the IBS properly, because that is what keeps pulling your levels down. Happy to review your reports on a Practo consult if you want a clear long-term plan.