Drinking alcohol loosens my bowels and makes me need the bathroom more often.
When I drink alcohol, I need to go to the toilet more often.
When I drink alcohol, I get an upset stomach and need to go to the toilet more often.
「お酒飲むと、お腹が弛くなりトイレが近くなる」＝ Drinking alcohol loosens my bowels and makes me need the bathroom more often. / When I drink alcohol, I need to go to the toilet more often. / When I drink alcohol, I get an upset stomach and need to go to the toilet more often.
ボキャブラリー
drinking alcohol ＝ お酒を飲むと
loosen one's bowels / get an upset stomach＝ お腹が弛くなる
need to go to the toilet more often ＝ トイレが近くなる
「お腹が弛くなる」は「loosen one's bowels」という言い方になりますが、「get an upset stomach」という言い方の方が丁寧で嫌なイメージを与えないと思います（笑）。「loose bowels」という表現は少し写実的すぎます！
Whenever I drink, I feel like I'm going to have diarrhea and I have to run to the bathroom.
この場合、次のような言い方ができますよ。
ーWhenever I drink, I feel like I'm going to have diarrhea and I have to run to the bathroom.
「お酒を飲むといつも、お腹がゆるくなってトイレが近くなる」
to feel like I'm going to have diarrhea で「下痢になりそうな感じになる」
ご参考まで！
イギリス 
カナダ 
