I have similar reactions every time I do a lemon juice enema. In fact I rarely do them anymore. You might want to switch to a simple saline enema (1-2 tablespoons of Seasalt to a quart and a half of water) for a while until you begin evacuating the enema water better. Enemas can be awkward and unpleasant at first, but they are really great on a fast for flushing out toxins. After your bowel is cleared you can go back to the lemon juice and see if you have the same reactions.
I am not an expert on juice fasting, the longest I've ever done is five days. However I hope my advice on enemas will help you.