How long a time did it take to see that gain ? In order to truly gain six pounds of fat would have required you to have eaten an extra 21,000 calories. one pound of fat = 3,500 calories.
So, if this gain is over say a two month period, it is possible to have gained six pounds of fat if you weren't mindful of what you were eating.
Now, if that same gain occurred in a week, then it's probably nothing more than a fluctuation in your water weight. I can easily gain or lose as much as 3-4 pounds in a day because of a shift in my water. Are you nearing your menstrual cycle ? Women typically retain water during their cycle. I can gain as much as 5-7 pounds. I don't let it freak me out because that's nothing more than water retention.
Could you have gained six pounds of muscle ? If you haven't been strength training or eating a surplus of calories, it's doubtful you gained six pounds of muscle. It really is hard for a women to increase lean muscle if they aren't engaged in a good strength training program and eating accordingly.
And well, not to get TMI, but when is the last time you had a bowel movement ? Our waste products have weight. So, if you don't do a poop before you weigh yourself, the physical weight of all the food you ate the day before is still in your intestines. Most people don't realize how much their waste products can weigh.
One day, for scientific purposes, I did weigh myself before and after a bowel movement. It difference was quite startling.
Should you worry about six pounds ? I would wait a week and see if that gain is water retention.