She sleeps MOST of the day! Just don't seem right? UPDATE

Please could someone give me some clue as to why my Sadie seems to sleep most of the day.

Some history:

My Sadie is now 15 months old, she weighs 49lbs.

She had her first season beginning of March '08, we had a bit of a scare here, thought she was pregnant. (false alarm) Phew!!

She has just been for her yearly booster jab and we also got her worming tablet, which I gave to her that day. (she has worming tablets every 3 months)
About 4:30am the next morning she jumped on our bed twice, she never gets on the bed until other half gets out to go to work, short while later I noticed she was quite distressed and pasting up and down the bedroom, then the aroma hit me, she actually had an accident! We didn't correct her as she did try to wake us TWICE.  Bless. Well the mess was quiet runny (usually she has nice hard stools) Would this be due to the worming tablet as we gave her a different one this time, (can't remember name off hand)

Well it was actually about Feb '08 we noticed her sleeping quite a lot, didn't worry to much as she seems otherwise healthy, eating, drinking, she's like a different dog when were out with her flick thrower, (she'll just keep running after that ball.) I spend about half hour to an hour with her up the field, I take her out 5 days a week. (2 days I'm unable to due to work pattern)

When we get home she'll have a little lay down then I'll give her her breakfast or evening meal. Then she would just sleep on the sofa most of the day. She has her little spurts in the garden with the kids and also when other half comes in from work! but  that's about it.

I had a Female boxer before Sadie and I know all boxers are different but she was quite active her whole life.

I'm just a little worried. Any suggestions as to why she's like this would help a lot.  

We do actually have a vets appointment booked for the 8th May '08 just to talk over any concerns I might have and about heart testing etc.. so I will ask vet when I see her as well.

Thanking you all in advance.