The post-upgrade slump is, unfortunately, a very common experience. There are a number of reasons why it can happen:
Sometimes what we call the ‘post-upgrade slump’ is actually just burn-out. You may have been doing extra work, or just using more mental and emotional energy, in the run-up to your upgrade.
After your upgrade, you usually then only have one more deadline – your thesis submission. Some people find the lack of tangible deadlines de-motivating, and like it’s an endless expanse of unstructured time.
However, other people feel anxious after their upgrade about the amount there is to do before they submit and feel like time is whizzing by.
Some people also start thinking more about what they’re going to do post-PhD, and start worrying about whether they’ll get a job or whether they’ll finish on time.