example: if you bought a stock at $100, it actually costs you $101. so if the stock only went up 50 cents, you're in the hole yet.
other things to consider:
did a stock you were holding drop in value over the course of a day? example: stock that was up $2.00 when you checked it at say 1pm may start to go down so that by 9pm it was only up $1.00.
did you have stocks on monday that cashed out? maybe you lost $ on those
i would definitely read the help topics and glossary links found at the bottom of the page