Not seeing your portfolio, I would say for a definitive answer, you need to click on the bug and ask WSS.
Offhand, I can hazard a guess or two ... one, do you have some shorts in your portfolio? Also, don't forget that any pending orders reduce your buying power. And if you have an outstanding loan, every day your money is reduced by the interest you pay on it, which also reduces buying power.
GOOD LUCK!