Ok. Firstly, joining a gym will help you tone up and change shape, but it might not help so much in terms of your actual body weight because you will build muscle as you train and muscle weighs more than fat. It WILL help over the longer term, but you may find that you are not eating enough to stop your body from feeling like it is starving. Serious athletes eat a LOT more calories than we generally use in daily activity. This in itself could impede your losses.
Also, being due on can result in a small temporary gain.
Look at the overall picture, you lost 4.5lb last week, it may be that your body is resisting losing because your diet isnt sufficient to counteract all the additional activity you are doing.
It is always the weeks where the awards are close where our bodies seem to resist doing what we want them to do. This is perfectly usual and all it means is that it is a reminder of the fact that it is not a race. If not this week, then you will get it next week, or the week after.
Do not be upset. We all have normal fluctuations as part and parcel of this process. Remember that your body shape will still be significantly improved by the time you get to the dress fitting. You say you are an 18 on the bottom and a 14-16 on the top. Even within a couple of months of doing what you are doing, you may well be the size you want to be, regardless of what your weight is.