Michael Strahan's daughter Isabella shared a disappointing setback amid her brain tumor battle ... revealing her next round of chemotherapy has been delayed.

The teen opened up in a YouTube video about her ongoing treatment ... explaining to her followers why she had to recently return to the hospital.

As Isabella put it ... a fever was to blame -- which is why she had to get an IV and have scans taken.

Isabella said she burst into tears over the IV ... explaining she was emotional since she hadn't needed one in such a long time.

However, this wasn't the only upsetting treatment ... she also had a needle stuck in 3 spots to drain fluid. She said she was awake the whole procedure.

Isabella underwent another craniotomy during her hospital stay, just one month after her second one -- which left her in an immense amount of pain.

She added ... "Not gonna lie, I’ve been crying a lot. I’m in a lot of pain. It’s not as bad as last time… my face isn’t as puffy but it literally hurts to do anything."

Because of the pain, the young model said she struggled to eat for days.

Isabella was eventually discharged after a week ... but found herself back in the hospital days later due to shortness of breath.

Isabella initially felt better after the treatments ... but learned they came with a cost. She said her next round of chemotherapy -- which is supposed to be her second to last round -- had been delayed for a month.

She said ... "I was just bawling my eyes out because it’s just so disappointing when you’re like, 'Yes! I can finish by May!' And then everything changes because of a stupid head infection."

Isabella first shared her medulloblastoma diagnosis while appearing on "Good Morning America" with her dad. Michael shares Isabella and her twin sister, Sophia, with ex Jean Muggli.

She's only 19.