Yes, but... Cortisone injections are not specifically a contraindications- but there are often non-invasive treatments for many of the same conditions. There range from icing to physical therapy to acupuncture. A physiatrist can help identify options for you.
But. But dequervain's tenosynovitis needs avoidance of wringing behaviors and splinting.
Yes. Cortisone shots or taken by mouth for that matter, appear to have no significant effect upon a breast fed baby or lactation.
Yes. Cortisone shots should have no effect on breast milk.
Do you also (as my doctor) see ok to get a cortisone shot and dimetinden therapy when breastfeeding (1 month old baby) due to a heavy nettle rash?
Probably. Some of the medications you take will get into the breast milk but given the severity of your rash, I think the risk /benefit ratio favors taking the medications since the medications would be present in only trace quantities and not likely to affect your infant. How did you get a nettle rash in an austrian winter? I'd have thought that an edelweiss rash would be more likely :)).