Do be careful that you're not pushing yourself with something wrong with you (I did that and got pneumonia - one of the side effects of that for months was my pulse rocketing up to 150 just walking up a single flight of stairs and ended up with a skipping heart beat for several years).
I have read that if you take your resting pulse first thing every morning (still in bed) and if it is around 10 more than normal, curb your exercise that day.
I've found a reference to it on the net - Exercise Tips - Exercise Ideas - Exercise Plans - Fitness Tips
scroll down to 'overtraining'.
So, if you suddenly find yourself in a 'dip' after previously being ok with a level of exercise, assess whether you have overtrained, caught a bug and are pushing yourself through it or whatever.
Don't only be excellent to each other, be excellent to yourself!