I found when my littlest got rashes (he has ear infections that burst his eardrums every 3 weeks, and was on antibiotics nearly constantly until 13 months old when he had grommets surgically implanted in his eardrums) the antibiotics made his poos horrid, and they burnt his tender bottom.
If it is fungal, either then you will need a fungal cream, you can get some good ones from the chemists that have a barrier cream in them, but if it is not spotty and fungally, then the best thing is nappy free time in some gentle sunlight, and i swear on johnsons cornstarch powder.
I tried all kinds of creams, but the corn starch powder actually keeps their bottoms dry, and allows their skin to breath beneath the nappy, so the skin can heal, even if their skin is bleeding, it works miracles.
All the best hun, it happens to every mother.