Sequencing inaccurate at the primer site
The very extremities of sequencing reads obtained by most if not all sequencing technologies are usually of lower quality, though more often so in the 5' region. You should disregard this data, or better yet design additional primers further away to encapsulate that region too if you desperately need it.

Below is a fairly typical output from FASTQC analysis of Illumina sequencing data. You can see how the quality is at its peak in the middle of the read (base index on the x axis. You'd likely see a similar thing for Sanger sequencing which is presumably what you're using.