I gained muscle, I hope?

I have been maintaining since Aug 2008 and have been very happy with my weight.  I actually lost a couple more pounds than I thought I would.  I'm 5'0" and 50 years old.  I exercise 5 days a week, I have a Polar hrm and I'm pretty consistant with my daily burn.  I had been weighing 102-103 since November 2008 and have done nothing different.  All of a sudden my weight has gone up to 105 and has been there for a month.  Is this normal?  I'm not eating any more than I ever have, or any less.  My exercise routine is always different, I have lots of dvd's and I change them out all the time. Sometimes only cardio, sometimes sculpting or weight training, sometimes both. The burn is always around 300 for about 60 minutes ,or about that.  I'm sure of this because of my hrm. My calories I consume are always about 1450-1550.  CC says I should eat 1550.  Where did this weight come from?  Is it muscle?  Should I eat more or less calories.  My clothes still fit fine.  My measurements are still about the same. It's not water weight because my weight would be fluctuating at least a little.  My weight is staying the same, not fluctuating more than .5 all the time.  It's not my scale, my husband uses the same scale and he's not having any problems with it. Well, that was long, sorry!  But my actual question is this muscle and do I eat more or less and do I worry about this ?  What do you think?  P.S.  I'm not going to exercise any more than I already do.  

edit:  I also watch my fat, sat fat, protein, carb, sodium and calcium ratios very closely.  I don't think that would be the problem.   

I think you've not been eating enough and that you've slowed down your metabolism a little - making it easier to gain weight.  I calculate that you need about 1800-1900 to maintain your weight given your stats (a 'moderate' exercise level)  

It's not really possible to build muscle on fewer calories than you need.  And, of course, the most usual reason for weight gain is 'eating more than you think'... so make sure you check-weigh food again for a while.

Thanks for the feedback.  Will I first gain more weight with this increase of calories, and if I do , how long before I start seeing a difference on the scale?  I'm all for eating more if that's what my body needs, I'm just afraid of overdoing it and not knowing if that's what I'm doing.  Other than the exercise I mentioned above, I'm really not a very active person.  I like to read, do puzzles, sew, work or play on the computer and such, and shop, but that's about it.  Just a ballpark figure of how long to not worry about the scale would be good.  I will keep an eye on my tape measure though!