A couple questions -

1. Is this the first time you are growing squash? If you've grown it before, did you have trouble before?

2. Do all your plants have it or just certain ones?

3. Check and see what the pattern of the yellowing is. Is it random and patchy, or are the veins staying green with yellowing in between (chlorosis)?

4. If the veins are green with yellow in between, is there browning on the edges of the leaves, or wilting?

If the yellowing is in between the veins with the veins staying green (chlorosis), that can be caused by several things. Over-fertilization is one of them. (Do you have a reason to think you over-fertilized?) :)

Iron deficiency can also cause it.

Another possible cause is really high soil pH (alkaline soil). You don't say where you are - some areas have naturally alkaline soil because of lots of lime. A soil test is always a good idea, you can get it done at your country extension office. High alkalinity can keep plants from getting iron, so it's iron deficiency again in real terms.

If it's low iron or alkaline soil, you can try some iron fertilizer. I have a patch of soil by limestone walls where lots of shrubs were chlorotic; last year I did a good application of iron fertilizer and they are growing great guns now with really good deep green growth.

Good luck!