19 days ago I switched from SSRI Fluoxetine to Sertraline as fluoxetine was no longer working (been on it 7 years at 60mg) and am now experiencing nausea, cramps and diarrhea quite badly. Is this normal. Should I stop Sertraline (on 50mg)?

No, it's not. normal and you could be having a reaction to the new medicine or the two medications mixing in your system . Ask the person who switched your medicines what to do, please.
Fluoxitine. If you "switched" from fluoxetine to sertraline without a gap of 10-14 days, the fluoxetine was still in your system, and added similar ingredients to the new SSRI, which can cause the symptoms described. There are SSRI's with a shorter half-life, but by now ("19 days") the fluoxetine should be gone and the symptoms should be decreasing. If not, get a change in SSRI's from your physician. .