Getting tired question [new swimmer]

Hi, I posted this on another swimming web site but then I found this one and it seems to have a bit more activity so I thought I'd post it on here as well. I'm a fairly new swimmer (been doing it for about 2 months) and although I have progressed quite a bit there is still one thing that I have a problem with which is getting tired. Here's what happens to me: When I first kick off the wall everything is fine, technique is good and breathing is good. In fact, I am going at a pretty decent pace and am having absolutely no problems. But at around 75m to 100m I seem to just hit a wall and all of a sudden I feel like I am swimming in a vat of molasses and going really slow. Eventually my form starts to break down and I find it difficult to get a breath. I don't know why this happens and was wondering if anyone else had experienced this or would have any suggestions on getting past this ? I don't think it's my technique because, as I said, everything is totally fine up to a certain point (i.e. stroke is good, I'm maintaining a pretty good horizontal position to minimize drag and breathing is good in that I am exhaling completely, or as completely as I can, underwater and getting a pretty good gulp of air when I do breath) and then it's like WHAM! I just start getting really slow. I'm also in good shape since I ski, mountain bike and hike so I know my lungs should be fine, but when I'm swimming I just reach a point where I get so tired I need to rest. Anyone have any suggestions.