What causes Helena to become angry with Hermia, and why does Helena refuse to believe her friend and would-be lovers?
When Puck accidentally puts the love potion on Lysander's eyes, Helena is the first person he sees, so Lysander falls in love with her and forgets Hermia. When Lysander showers Helena with oaths of love, she thinks he is mocking her because she chases Demetrius. Then, after Puck and Oberon figure out the mistake, Demetrius gets the love potion on his eyes. Once he starts declaring his love to Helena, she believes that the two men have teamed up to mock her. Finally, when Hermia enters the scene completely confused as to why Lysander loves her best friend, Helena says the following:
"Lo, she is one of this confederacy!
Now I perceive they have conjoin'd all three
To fashion this false sport, in spite of me" (III.ii.193-195).
This is why Helena is angry at Hermia. She believes that all three of them are now making fun of her. As far as the "would-be lovers," Demetrius and Lysander are fighting over Helena and vowing that they love her all at the same time. This is difficult for Helena to believe because it wasn't too long ago that Demetrius was hoping that wild beasts would do her harm in the forest and Lysander was eloping with Hermia. In an ironic turn of events, the men are now all of a sudden in love with Helena. She can't conceive any other option than that they are all conspiring to make fun of her plight. Hermia, Lysander, and Demetrius don't understand Helena's anger because none of them have any idea how things got so twisted.
Helena is angry with Hermia because Helena loves Demetrius, but Demetrius, in the beginning of the play, loves Helena. In Act 1, sc. 1, Helena says that Hermia is beautiful and has a sweet voice - "Your eyes are lodestars and your tongue's sweet air / Are more tunable than lark to shepherd's ear..." and that is why Demetrius loves her. Furthermore, Helena accuses Hermia of using her feminine wiles to gain Demetrius' love ("O, teach me how you look and with what art / You sway the motion of Demetrius' heart.")
I'm not certain what the second part of your question is asking.