At Once
At One Time
adverb
Remember how you learned that 一遍 means "once?" Well if you add に and make it an adverb, it becomes at once.
いっぺんに
This has the same reading you learned with 一遍 so you should be able to remember this too!
一遍にいろんなことを言われても、分からないわ。
If you say a bunch of things at once I won’t understand.