Mild cramping can start as early as week 4 or 5 as the uterus begins to stretch. However, if the pain is very severe or accompanied by bleeding, please contact your gynac immediately.
Mild cramps are normal in early weeks, but keep track of them. Don't ignore severe pain, as emergency hospital visits and tests can get quite expensive, so call your doc early.
I felt light period-like cramps around my missed period date and got so scared! My doctor reassured me it was just implantation and normal stretching, as long as there is no spotting.
With IVF, we tend to notice every single twinge. Mild cramping in the first few weeks is very common, but please do update your clinic for peace of mind.