Shrinkdude makes some good points.
I do love the concept--"woman shrinking while uncontrollably horny" is probably one of my favorite things, and I never really get tired of seeing it.
The FX do have a few rough spots, although I'm sure that will improve with practice. And they're better in the second half of the video.
As he said, the use of the larger bra falling off was a very nice touch. Oversized clothes falling off are always welcome!

And kudos to you for keeping the clothes onscreen lying next to the shrinkee; it tends to be a bit jarring when they just vanish.
Your performance is decent (certainly better than a few I've seen).
The sinister atmosphere isn't really my thing personally, but I understand what you were going for here.
Overall, it isn't bad, especially for a first effort. I'd certainly be interested in seeing you do more.