Dogs - Giving Prednisone nad Rimadyl on alternate days

My DVM has prescribed 10mg Predinsone (for inflammation - bronchectisis) and 75mg Rimadyl (pain for
ostoathritis) for my 80-lb, 13.96 -yr old alaskan malamute. The Rx's
are to be given on separate days (Mon-Pred, Tues-Rim, etc). I was
scared about starting this but did. My dog began this on Mar 30, 2011
and ended on April 12. Mookie came down with acute abdominal pain on
the morning of April 13.

Is this enough time and too high a dose of
Prednisone to cause ulcer/peritonitis? No dark stools seen. I started the
Prednisone first then worked in Rimadyl. Mook ad a total of 7 Pred pills and 3 Rim pills, with 5-day wait for last dose:

Mar 30 - 10mg Pred
Mar 31 - no Rx
Apr 01 - 10mg Pred
Apr 02 - no Rx
Apr 03 - 10mg Pred
Apr 04 - 50mg Rim
Apr 05 - 10mg Pred
Apr 06 - 75mg Rim (noticed some confusion, so decised to stop Rim after
this dose and continue Pred)
Apr 07 - Pred
Apr 08 - no Rx
Apr 09 - Pred (noticed he stopped walk short, began thinking on trying
to add back Rim)
Apr 10 - no Rx
Apr 11 - Pred
Apr 12 - 75 mg Rim (did not help, his evening walk, rear legs, went
1/16th mile at most)
Apr 13 - crying in backyard, could not get up, 104 F fever, when I
lifted and carried him in, he sounded out as in pain.

I am trying to understand if I should never try this again. The DVM
taking care of Mookie assured it was ok and that he has
done it several times with dogs.

I have a heavy heart today...

Mike