So I was assuming that the Tigrinya people, if you go far enough back, are actually Arab/Congolese mix? Where the Arabs migrated down into Eritrea while the peoples from the Congo were migrating up to Northern Africa?
Not true. The majority of the Eritrean people have been in the highlands for thousands of years. There is a noticeable difference between the unmixed Ethiopian/Eritrean gene and the Congolese gene. Here is a study where they address the unmixed Horn of African gene (Ethiopic), the unmixed Near East African gene (nilo-saharan), the unmixed West African gene and West Eurasian Admixture.
http://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1004393It shows how diverse Africa is and how of the West (niger-congo), Near East (Nilo-Saharan) and Horn of Africa (ethiopic) are very unique from one another.
Now as to where your mothers results might have came from. There is a group in Eritrea that migrated from West/Central Africa that resides in the Western regions.
Its very possible that this could be the case. In any case I hope it doesn't make you feel less Eritrean. Many African Ethnic groups migrated from east to west and vice versa. You are still Eritrean no matter what the tests may say.