Spay scar has reopened

[NotTheDuchessOfCambridge]

We had our dog spayed about 6 weeks ago. We followed the vets instructions to a tee and in her 2 week check up the vet said she was healing fine. Fast forward 4 weeks and I have just noticed the wound had opened by about 2ml and there is a pea sized lump underneath. I have put her post op baby grow back on her to stop her licking it but would this now require a trip to the vet?

There's probably a reaction to one of the internal sutures. If it doesn't settle within a day or two then it merits a trip to the vet, but it's not an emergency unless it's very red/sore/oozing.