Overdoing it?

How do you know if your overdoing it on training? I started swimming again last Tuesday after many years off. I did a short 10 minute out and 8 minute back swim that I figured would take me 15 minutes each way. I was very surprised. The next night I did a 17 minute each way swim going further then the previous night. The 10 minute turnaround point the night before, a bouy only took me 7 minutes on Wednesday night. Thursday night I did a 1.6 mile swim, 32 minutes out and 34 minutes back. This time it only took me 6 minutes to make it out to the first bouy. All the swims I have done thus far has followed the same route, they have just went further along the shoreline each time. Friday night I swam out and it took me 31 minutes to make it to the turnaround point from Thursday evening. I continued on and extended the swim further. I turnaround at the local yacht club marina. I had 45 minutes out and 52 minutes back for a 2.4 mile swim. Sunday I decided to go for the biggy thus far. It was going to be a 3.7 mile swim roundtrip. I knew after this I was going to start running into problems with refueling so I knew anything further north than this particular point would require some added strategy into the equation. I got to the lake and saw the water surface and was shocked. I didn't think the wind was blowing that much. I knew this was going to be the roughest water I had swam in thus far. I still managed to meet or beat each of the way points on the way out to the turnaround point from Thursday night until the very end. I managed the 31/32 minutes in 32 minutes. I headed on up north along the shoreline and reached the yacht club in 44 minutes. I was surprised. I reached the turnaround point in 1:10. I got back to the yacht club and had the big surprise awaiting me. The lake had kicked the roughness us a bit. I pretty much swam *** stroke along the yacht club for safety. I didn't want to find myself getting tangled up in any of the anchor lines on any of the boats. I mananged to get back to the finish point in 2:28, right at a 1.5 mph pace. I was still feeling fine, better than I did Friday night when it almost seemed like I had a bit a 'motion/sea sickness'. It was the weirdest feeling I've ever experienced. Thankfully it only lasted 20-30 minutes. How do you know what your overtraining. I could go out tonight and swim another 2-3 miles and not think a thing about it. I do have other obligations that are going to keep me from doing it. Right now I'm looking at a 4.2 miler possibly tomorrow...depending on the weather and maybe a 5 miler on Wednesday, taking Thursday off to get ready for a much longer swim on Friday, maybe 8-9 miles. I have to fully work out logistics of the refueling before I even think of trying the longer swim. It just seems strange that I haven't hit any kind of brick wall on any of the swims yet. I would have thought I would have noticed something that would suggest that I have overdid it. The only thing would have been the Friday evening 'motion sickness'. My pace is pretty much consistent throughout the swim unless I hit rough water conditions like I did yesterday and I get pushed by the waves on part of the swim and then pay for it on the return leg.