what you are doing is fixating. a very natural reaction. A mild example of your condition is people who struggle to meditate because they are "itchy" now i've put that out there the next time you try to meditate you will feel a mild itch which moves around your body and irritates you out of the meditative state. why is the brain doing this? it's a resistance, a fight against the loss of self control. the inner self fears what it does not know about. like a dark ally. what will happen if you meditate deeply? what will you see where will you go? these questions are very deep and personal. fighting against the unknown is natural and your physical conditions of pain are only giving your mind a crutch to lean on and avoid the inevitable. the inevitable is that you will enter a deep state of meditation, you will experience the itch i have mentioned and then enter a deep state of meditation.

Why is the brain experiencing this fear? the physical part of the brain which activates when we experience "paranormal" experiences is located next to the part of your mind which experiences fear. The most common people who experience what you are going through are those who have experienced a paranormal phenomenon at some point in their life. typically a misunderstood childhood event. dreams as a child are often very complex and confusing.
This has caused a kind of bruise, an irritation in the mind which activates as you enter meditation.
how do you overcome this? 2 ways, the common socially accepted solution is very similar to OCD treatment, so you can look into exposure therapy if you want.
the other solution which i would recommend. when you are feeling the pain i want to you to mentally leap to a child hood memory. Any child hood memory will do. think about that event and within moments the actual event from your childhood which has caused a fear of the unknown will leap to your mind. just recall that event instead following the relaxation instructions that you are familiar with. dwell on the memory and relive it. as you do the meditation will deepen and you will be over it.

best wishes.