He may be feeling vulnerable due his age and health status.

My border collie who is also 11 (12 in November) has also become grumpy. For no reason he will attack my little red and white border collie. Not enough to harm fortunately.
He has an enlarged heart and is becoming a little off his back end.
He only does it when i am around. He's always been a mummys boy. I've put it down to grumpy old man syndrome

Hi Littlepony,

Let's look a little bit at his known health issues. You mention he has IBD, but then changed it to IBS, these are two different conditions.
Many people confuse inflammatory bowel disease with irritable bowel syndrome, a stress-related diarrhea problem. Treatment for irritable bowel is aimed at stress; it is a completely different condition from IBD.

The drug you are giving him has the following side effects:
side effects of Carprofen (Rimadyl) include:

Black, tarry stools or flecks of blood in the vomit
Drowsiness
Staggering, stumbling, weakness or partial paralysis, full paralysis, dizziness, loss of balance.[8]
Change in urination habits (frequency, color, or smell)
Change in skin (redness, scabs, or scratching)
Change in behavior (such as decreased or increased activity level, seizure or aggression).[9]

How long has he been on this drug? I would tell your vet about his recent behaviour and ask if he can be prescribed something else, maybe metronidazole?

IBS or IBD?
http://www.veterinarypartner.com/Content.plx?A=598