I am one that lost my girl to lymphoma and heart complications too. Just a bit over 5 months ago Dec 26, 2005)
My girl had 6 weeks to the day of the dx. She was happy right until the end. No one could believe that she was loaded w/cancer, she acted as she always did..puppylike. I was lucky, she never displayed any signs/symptoms of cancer ie:runny poo, lack of appetite, vomiting etc.
I miss my girl terribly but cherish each and every second of those 6 wks that I had.
All I can offer is make lots and lots of memories, take lots of pictures, do all those things you have wanted to do w/her, and do all those things that always made her happy, feed her whatever she wants..within reason

, remember you will probably have more GOOD days than bad. Try NOT to cry etc in front of her..ohhh they pick right up on that and it tends to drag them down. Don't waste time being angry, sad, teary etc..instead take that time and love that baby!
I am soo very sad for you. I know what you are feeling..as lots of us do..and its a heartwrentching feeling :(
Sending out positive vibes to you and your girl