Neil
I'm assuming that your sunday run is your longest and that you usually do this at a faster pace than some of your other runs. If so, it can often be the case that the effects of a longer/harder run will be felt more strongly two days later, rather than the next day. Hence monday's run possibly seemed easier than expected. Going a bit faster than intended on monday,combined with the sunday run, probably led to the fatigue on tuesday and wednesday.
Increasing weekly mileage can be quite challenging, so when doing so, maybe you might want to have two easy days after the long run. Even if you feel you can go faster, it it is probably best to stick to an easy pace on these two days. Then, if it feels okay, look to pick up the pace on the third day(wednesday), followed by another easy day.
Although you have had a few bad days, you will probably learn a lot from the experience and in the longer term even the bad days will be of value (that's certainly what I have found!).
I hope the above is of some assistance.