Well done on the gym! I've got my first session in three weeks next Tuesday... (holiday and knee injury have stopped me from going) Looking forward to it already!
On the putting on weight or the slowing of weight loss, I think it's because just after exercising, especially with weights, your muscles will retain some water, and also muscle weighs heavier than fat when you compare it per volume. So it may seem your weightloss will slow, but if you check, you'll probably lose lots more inches! And the weight will come off, I promise!
I have to say, I'm a tight size 12 to a small 14, and I weigh 12st 10lb. I'm 5'10", so realistically I think I should be a tight 14 or a small 16, but because of all the exercise that I've been doing over the past 9 months, I think that my body may weigh heavier, but my fat % is much lower than it was when I wasn't working out and thus my body is smaller...
That said, I still want to lose some more weight, LOL!