YOU SAID:

i was going to go to the party, but I had a headache

INTO JAPANESE

パーティーに行くつもりでしたが、頭痛がしました

BACK INTO ENGLISH

I was going to a party but I had a headache

INTO JAPANESE

私はパーティーに行っていましたが、頭痛がしました

BACK INTO ENGLISH

I was going to a party but had a headache

INTO JAPANESE

私はパーティーに行っていたが、頭痛がした

BACK INTO ENGLISH

I was going to a party but had a headache