We use these phrases in different situations.
Please imagine.
You have a plan to go to somewhere in order to enjoy shopping with your friends on this weekend.
Maybe you are looking forward to the day.
Also you had a big Japanese language test on last week.
You are confident very good score for the test.
And you will receive the results on the day after tomorrow.
Maybe you can't wait for the day.
This feeling is 'waku-waku'.
わたしは、ワクワクしています。
You had a big Japanese language test on last week.
You have a little confidence for the results of test.
And you will receive the results on the day after tomorrow.
You can't forecast the score,you are afraid bad score.
Also you are sitting on the bench,and the very handsome man is sitting next to you.
You have been loving the man very much.
You may feel your heartbeat in these situations.
This feeling is'doki-doki '.
わたしはドキドキしています。
In fact,doki-doki is the sound of the heartbeat in Japanese. :)