This is a relatively common occurrence for individuals who are changing from a heel strike form to a mid of forefoot strike form.
My first question for any runner who is changing form is: why? There are several reasons that runners will often give such as injury problems, advice from runner friends, performance, reading about it online, and many more. Ultimately, I’m a proponent of only changing form when it “needs to be changed”. Over our lives as children into adults, we all develop a natural and personal running form. Some of this form is taught, but most of it is the natural way our body decides to move. Over this same course of time, our body tends to become proficient in this running form – it expends as little energy as possible to move you as far/fast as possible. These proficiency changes largely occur in neurological adaptations along with very specific and local musculoskeletal adaptations. For example, as a runner completes more running, certain parts of a bone will be reinforced based on the amount of force that part of a bone has to absorb. Over time, the bone will become “stronger” in the location that is stressed by the running.
Form change has been a big deal over the last few years and has actually stayed around a bit longer than the big wave of minimalist/barefoot running that was very strong a few years ago. Sometimes form change can be positive and needed, but other times it can be detrimental and could lead to injury. I once worked with an individual who had to stop running for several years due to chronic calf/achilles issues. This runner decided to make a comeback and wanted to have help, so he hired a coach. The coach was a huge proponent of forefoot striking and encouraged this individual to adopt that pattern. Forefoot striking has been shown to increase the load on the calf/achilles, and the runner suffered from even worse achilles pain during his comeback. This is an example of how form change can go wrong.