The day of the surgery we dropped her off and worried ourselves sick. Then another vet called up to say that the lump had been a haematoma and there was no point sending it off cos there was no actual mass, just blood. He said we could come and collect her that afternoon and we were very relieved.
We got there to pick her up at 2:30 and were left waiting for about an hour before being called through to talk to the vet. It turned out that as they were getting her ready to go they noticed that the area had refilled with blood again, which they hadn't expected to happen. He also said that she bruised very easily and he was concerned she may have problems with blood coagulation. They did a scan of her liver and spleen, which were both normal, and took blood samples to send off.
We went back the next day for the results. Her red blood cell count was slightly high, but not alarmingly so, and could have been the result of her spleen contracting due to nerves when they took the sample. The lab hadn't been able to test for coagulation issues because the sample had coagulated in the tube before it reached them! Another sample was taken and that came back with no issues found.
The vet suggested a surgery to remove the void that the blood is filling up in in order to get rid of the lump, but we decided that if the lump isn't causing any problems and isn't a danger to her then we don't want to put her through another operation. He then consulted with a specialist who said that it might be due to a vascular anomaly or a tumour and we should wait a few weeks, keep an eye on the lump to see if it gets any bigger, and then bring her back for another appointment to see what the situation is.
At the moment though she seems very happy and bright, which is what's important. And I have to accept that she is getting on in years. If it does turn out to be something serious I don't think I'd want to put her through lots of invasive treatments. I'd rather have a happy dog for a shorter time than make her miserable just to keep her around a bit longer.